Hi, tried all options and they don't work. I did fix my macbook ro by updating to Big Sur, but I cant update my imac as my audio interface is not Big Sur compatible.
This is starting to become a problem for a lot of people, so it seems odd we all have the same issue, so its an App store problem that has only just surfaced since about 4 Feburary.
So the next problem I encountered, following your tip by trying to reinstall Mojave is that I dont use an apple mouse. And my logitech mouse would not connect in this restart option of Command-R. Somehow, buy using the keyboard, I managed to navigate through quite a few screens until I got to the last one (typical) Made it all the way to 'select which hard drive' to install OS, But none of the keyboard buttons would highlite Macintosh HD and the screen just sat there and I had to close down the session.
So basically I couldn't reinstall the OS.
Driving me crazy. Any other ideas.