Computer is stuck in loop after trying to install updates

Update popped up, so I did. It's been a while. Got a message afterwards it could not update this computer. Now, every time I start up the computer it tries to update. I will not get out of this loop. I just want the old system back. It was fine before I clicked update. I believe it is OSX 10.13.6. Its a intel mac Pro.

I tried hold "D" down for dianostics with no luck.

Can someone help? Thank you