Revert from Catalina to Original OS (Yosemite) or can I just revert to Mojave

Thanks for your quick response. The company says they are working on an update to the program, but has been saying that for most of a year now (since Catalina was released). Catalina has only been installed a few days and really nothing has been done on the computer since other than email which is recoverable due to IMAP account. I have already read that support document you linked to and followed the instructions to try and restore from Time Machine. However, I get an error that says "This system can't be restored onto this disk because HFS systems can't be restored to space sharing APFS volumes. Reinstall macOS on this disk and then use Migration Assistant to transfer data from your backup instead." I guess we just need to revert back to original OS and then could update to Mojave.

I got this error message when trying to restore from the last time machine backup prior to Catalina being installed. "This system can't be restored onto this disk because HFS systems can't be restored to space sharing APFS volumes. Reinstall macOS on this disk and then use Migration Assistant to transfer data from your backup instead."

I did try booting into recovery mode using the Shift+Option+Command+R which is supposed to let you restore using the original OS which would have been Yosemite. However, when trying to do that, it did not show the Macintosh HD as an option to restore to. The only disk that showed was the external drive which is the Time Machine.

So, once into Recovery mode, do I then go into Disk Utility to do the erase? I assume I then have to get an OS installed before I can use migration assistant to restore files/apps from Time Machine? Or can I just do a restore from Time Machine once the erase is done? Would that get back the OS that had been running before Catalina was installed as well as all the apps/files?

Thank you for your help. I will give all this a try and see what happens.

I just wanted to tell you I followed your instructions and was able to get the computer erased and restored from the Time Machine backup. Thank you again.