Photos and Facebook

You need to do it from the Facebook end of the chain with your browser. That is until Facebook provides an extension to allow direct upload from Photos Library.

The browser window will look similar to this:

Then it's something else that's causing the problem. Try the following:

1 - log into another user account and see i9f the problem persists there. This will tell us if the problem is limited to your user account or is system wide.

2 - boot into Safe Mode and see if the problem persists. This will tell us if the problem is caused by any 3rd party software.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.

3 - if the problem persists in Safe mode download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

Before running Etrecheck assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report:

Then we can examine the report and see if we can determine what's causing the problem..

I've tried this, but no photos appear. Photos is open. Any suggestions? Thanks!

Where is your library located?

Make sure the library is designated the System Photos Library in Photos' General preference pane:

I appreciate your help, and now I'm really confused. If it is set to see them, why can't I?

SN: I can drag a photo (jpg) to a different folder and can see it just fine.

It is grayed out. But now I’m not sure how to change it.

Did you read the text in the screenshot I posted?

I did, but when I go to Preferences, I can't change it to the "Use as System Photo Library" option.

If it's grayed out it already is designated the System Photo Library.