Many thanks to all of you trying to help me.
Answering TomH: (1) The only auto backup or other process accessing my *.rmgc db file that I am aware of is my cloud backup, Google Drive. I have tried my RootsMagic Backup many times with Google Drive on Pause and also when sync is stopped. Backup still won't work. I get the same error message. (2)) I have never heard of SQLiteSpy or SQLite manager. I don't even know what they are. (3) I don't know how to look for something that has the database file open. (4) I am not running any concurrent versions of RootsMagic. The latest v. 7.5.7 is the only one on my desktop computer. I should also point out that I have the exact same v.7.5.7 and *.rmgc file on my Windows 10 laptop, and Backup works perfectly. The difference is that I do not have Google Drive on my laptop.
Answering kbens0n: (1) I followed your suggestion, but Backup still won't work. I am absolutely positive I have not had more than one copy of the database open at the same time in the recent past. (2) I will try to figure out how to run and interpret the Handle program. I am an older guy, but fairly computer savvy. However, I am certainly no computer nerd/geek.
Thanks again for your responses.