Updating...but then nothing happens?

I don't have any antivirus apps etc since I trust Apple haha! I've never had these issues with my Pro / my other Air. I've tried installing in safe mode, another user mode etc. But nothing seems to work :(

( But thank you for answering & helping!)

I might also add this, in case it gives away more info what may be wrong:

"I've done the steps multiple times and everything goes smoothly - until the laptop is shut off & turned on again (by itself). Each time the line surpasses the end point & should be finished, I log into my laptop just to see I still have Catalina.

I cannot update minor stuff either, since the laptop shuts off, the bar finishes all the way, turns on & I get a message saying " You have a new update" (which is the same update I tried to update haha)."

This is so weird since I've tried to update for at least 2 weeks now without anything happening... but I decided to give it one last go before reporting and suddenly, my Mac has updated to Big Sur?!? I don't know what happened, I did the exact same steps as before but somehow it worked after the 100th try. Everything seems to be working fine and I finally have Big Sur, yay!

Thank you!