Big Sur OS X

I have a late 2013 iMac but use an external SSD boot and store drive. Trying to update to Big Sur, app store says it can't find it. A security update keeps indicating request to be updated, but on start up it tries to go back to the internal old mac drive instead of the boot up ssd. I can't write code! It wasn't a problem last year when I updated to Catalina.

NVIDIA GeForce GT 750M 1 GB

2.9 GHz Quad-Core Intel Core i5

16 GB 1600 MHz DDR3

Catalina 10.15.7 (19H15)