Main internal drive unable to mount

Hi there, need help please... Start from the beginning, these are all the things I did.

I restored my MacBook to factory settings then reinstall El Capitan, now I'm waiting for the macOS Catalina to finish installing. Oh by the way all the macOS updates were installed on my external drive as  there wasn't an option earlier for me to do that in my main internal drive (can't see it). Now as I'm finishing installing mac OS Catalina, internal drive is not visible when I looked up in start up disk. When I checked disk utility and click on first aid, it doesn't mention problems just scanned and all done. The "Mount" option is not clickable. I looked up on web and it's kind of confusing. Is there a simple way to do this? Thanks for your time.