Tcolorbox tcbcol@back and tcbcol@frame are undefined in 4.20

The internal color names became official with version 4.20. Therefore, the @ was removed from the old names.

Here are the offical names (from the documentation):

Looking at this answer and comparing this to the updated version of tcolorbox.sty in which you can find

...
colback/.colorlet=tcbcolback,
colframe/.colorlet=tcbcolframe,
....

suggests that you need to use tcbcolback and tcbcolframe instead.