Still having error reading cache message

You are getting good answers from @GoldingD  and to be specific about your questions-

"Under an icon titled "LR catalog previews LR data there are two files: "Preview.DB" and "Root pixels.DB". Are you saying I should delete one of these files?"

Not exactly. You should Delete/Remove/Send to Trash the entire 'Previews' package/file, that includes the two .DB files you are referring to .  ie. Delete/Send to Trash: { catalog Name Previews.LRDATA }

What is the difference between deleting and sending to recycle bin?"

None. Different terminology for PC and Mac systems.

Some added advice to the posts from @GoldingD :

It seems you followed the instructions at PREVIEW CACHE ERRORS GRAY PREVIEWS 

And you would be correct in deleting a { Catalog Name Previews.LRDATA } file, BUT ONLY that specific file!.

When you re-open a catalog (by [double-clicking] an .LRCAT file), all the previews in the .LRDATA folder are slowly re-built in a new  { Catalog Name Previews.LRDATA } file.

You also state- "I hit the "catalogircat lock " icon "  The file with a .LOCK filetype suffix should only ever be present when a catalog is open and operating. If you try to open a catalog when its .LOCK file exists it should throw up a message like 'the Catalog is open in another app'.  This is a different problem to the corrupt Previews cache, but is simply corrected by deleting the {catalog name .LRDATA.LOCK} file and ONLY the .LOCK file!

It is sounding more like you did in fact delete your primary Catalog file. And the only way to restore 'it' is to find and open the most recent backup of that specific catalog.  (I don't remember if Lr5.7 created ZIP backups but simply saved a copy of the .LRCAT file.)

1)If I can't get into my primary catalog, is there some way to make my backup catalog my primary one?

had to get back to you on that. And perhaps another member can speak up (a bit surprised they have not stepped in on the other reply just yet, hopefully they will provide an improved reply)

So, normally the backup, and I hope you have multiple backups, when you need it, is processed as follows (their are variations)

1. Use your OS File Manager (In this case Windows File Manager) to locate the catalog that is giving you issues.
2. Move that catalog to another folder, or rename it.
3. Locate the hard drive your backup catalog is on (hopefully not the same hard drive as your working catalog) And then the folder the backup is in, and finally the backup catalog.
4. Copy the backup from the backup folder to the desired folder. The folder the catalog that is giving you trouble was in (or got renamed)
5. Decompress (Un zip) the backup, the copy
6. Double click on the catalog
7. If the name of the catalog is the same as the previous (damaged) one, and the file paths are the same (reason to copy from the backup location to the working location) Then the catalog should not have issues with missing folders missing images.
8. Continue with it

If your LrC /Preferences/general/ setting for the default catalog is use most recent, then this catalog will be opened next time.

The backup catalog you are now using, may not be up to date, depends on your backup regularity. If images are missing due to catalog date, you could try to import them from the damaged catalog.

The concept of a Primary Catalog is a user mindset. It can be just the catalog you normally do work in as opposed to say a test catalog. It could be a main catalog you keep on your Desktop computer, as opposed to one on a laptop you take on travel, and then bring over the work in the laptop catalog to the desktop catalog. Whatever. LrC will open, either the most recent catalog used, a specific catalog, or it will prompt you, all based on that one setting in /Preferences/General.

P.S Backup strategy, 3-2-1

The 3-2-1 backup strategy simply states that you should have 3 copies of your data (your production data and 2 backup copies) on two different media (disk) with one copy off-site for disaster recovery.

Hard drives, NAS, and the Cloud come to mind. And never ever keep a backup on the same drive as the working copy

• https://fstoppers.com/business/youre-probably-not-backing-all-your-lightroom-classic-data-526597

2)Does the "open recent" option continually list all the catalogs I've used, or do some get erased if by mistake you did something stupid in LR. I'm asking this because I don't know the name of my primary catalog and am wondering if there is some way to find it.

Ah, I rethought my reply. The Adobe option Open Recent, (actually it states Load most recent catalog) does not refer to multiple recent catalogs, it refers to the most recent catalog, or if you prefer (grammar wise) the previous catalog.

Now, you could after opening a catalog, use the LrC menu bar, click on FIle, click on Open Recent, and you may see several to pick from

You have windows, so I can remember the keyboard sequence to start LrC in an alternate opening. Another way to start LrC, is to use a combination of your keyboard and a LrC startup icon, hold down on the ALT key, and while holding that down, click, or double click (as required) on the LrC startup icon. And you will be presented with the following:

From that screen, you can select the recent catalog you desire. Or you could create a new catalog (creating a new catalog this way can be used as a diagnostic if your catalog is funky)

Incidentally the above pop up screen, The LrC prompt, is the screen you will see, if in LrC /Preferences/General/ you select Prompt me when starting Lightroom

Now a point about how to find LrC catalog that are on your hard drives. The catalog or catalogs will have the file extension .lrcat. In the above screenshot you can see several. Now for my example, I selected my test catalog that you see listed above in that prompt screen. Using Windows File Explorer, I see this:

That file I have circle is the catalog. The one ending in .lrcat. Not the others like .lrcat.lock, lrcat-shm. .lrcat-wal. I could have had additional catalog, or copy's of the same but older versions of this catalog. Point is the file ending with .lrcat is a catalog.

If you do not know where you placed various catalog, you could always use the Windows File Explore, and search on .lrcat.

note, some members will scream at me as an old goat keeping multiple catalog. All you need is one. Well as they say in the South, “Aren’t you precious”. Yes old goat old methods, not going to change. I like it that way.

Use your Windows File Explorer and select the hard drive, and then the folder that your catalog is in. The catalog will have a .lrcat suffix. Double click on it.

Was the error message "reading cache" or "reading catalog"?

Catalogs are files with the .LRCAT file-type suffix. Did you delete any .LRCAT files?

An error "reading catalog" would indicate a corrupt catalog. If it will not 'restore' then you can only recover from a backup of the catalog (or use another catalog).

Lr5.7.1 is so old now I don't remember what you might be meaning by- "--root pixelss and db files.