Problem with LR upgrade (ver 9.1) States need to upgrsde preexisting catalog for use with LR Classic

If LR v9.1 is asking you to update a Catalog, then there MUST BE at least one catalog it is aware of on your computer.

If really there are no catalogs on your computer (or they are undetectable)- LR will simply create a new empty catalog for you to import photos.

Any major upgrade of Lightroom-Classic (eg. v8 > v9) involves a change to the database structure of the catalog. This does require a catalog update: A copy is first made, the catalog is updated with '-2' added to the name, LR 9.1 opens and uses the '-2' catalog.

And "cannot be undone" simply means that the new '-2' catalog cannot be used in older versions of Lr-Classic, ie, not "backwards compatible". (you do still have the older version of the catalog that was copied!)

Not true. Whether you have a catalog on your system or not, IE NO Catalog at all.  LR Classic 9.1 will still open, or should open, and if no catalog is found it will create a NEW BLANK catalog.

In fact you should only get the prompt to upgradse a catalog IF a catalog is found on your drive. Could be that it also looks to the older version of LR for the location of the current catalog to upgrade. But anyway you can Cancel out of the upgrade dialog system and start LR with a new blank catalog.

What if you never used LR Classic, the Traditional version of LR, before and never has any catalog or previous version installed. LR Classic would start, create a new blank catalog (this if it could, IE No folder/file permissions problems) and then ask you if you wanted to import images.

So post what exactly is happening on your system.