Sandra
Go download the db for Nested Subforms.

It is still working fine for me (I'm still on 32bit BTW).

If it works for you in that one, you may have corrupted your db some how.
Back up what you have right now.  (You are backing up any way - RIGHT?!)
Compile your db in the Code Window.  Compact and repair.   Run through the  Troubleshooter

Yeah, working fine for me on a clean DB. O365 x64

I agree with Scott. Repair Measures.

Try compile your VBA and C&R. Failing that, try Decompile and Recompile (This has worked for me in the past when just compiling didn't).

All failing that, try the nested cont forms on a clean DB to see if it is an issue with our office installation.

I want to thank both of you.  Luckily I have a split database, and I tried the last saved front end, and it is now working.  Super strange, but glad to know I can still use 2019.  I never thought about going this route. Thanks again, y'all are awsome.