Can't save in JPEG format, nor open pics in JPEG anymore

Just to have it out of the way: You cannot save to jpeg unless the document conforms to the jpeg specification: 8 bit depth, no layers of any kind, no transparency and no alpha channels.

A common gotcha is a setting in the Crop tool to “hide” instead of “delete” cropped pixels. The former produces a floating layer, which is not allowed in the jpeg file format specification.

However, this looks more like either corrupt preferences or a corrupt install.

First try a reset of the Photoshop preferences: https://helpx.adobe.com/ie/photoshop/using/preferences.html#Manually

Remember to back up your settings before doing the preference reset: https://helpx.adobe.com/ie/photoshop/using/preset-migration.html

If this doesn’t help try to manually removing the preferences files. This is the most complete method for restoring to the default state. The manual method ensures all preferences which may be causing problems.

Please follow the steps below:

1. Quit Photoshop.
2. Navigate to Photoshop's Preferences folder.
   
   MacOS: …Users/[user name]/Library/Preferences/Adobe Photoshop [version] Settings
   Windows: …Users/[user name]/AppData/Roaming/Adobe/Adobe Photoshop [version]/Adobe Photoshop [version] Settings
   
   Note: The User Library folder is hidden by default. To access the content in the hidden folders see here:
   
   MacOS: Access hidden macOS library files
   Windows: Show hidden files, folders, and filename extensions in Windows
    
3. Drag the entire Adobe Photoshop [Version] Settings folder to a safe place to a backup your settings
4. Open Photoshop.

 New preferences files with the default settings will be created automatically.

There’s a update to Photoshop 27.7 available. Download and install it and check if it’s help.