Test if no application

This config loads if Picom is available – OK.
Test (x picom) Read $[USER_DIR]/SimpleButton/simpleB.sys

Without Picom, how to test it — ???
Test (None picom) Read $[USER_DIR]/SimpleButton/simpleB-nop.sys

Multiple ways, first is just negate the test with !:

Test (!x picom) DoStuff

If you happen to be inside a function and you want both of these possibilities you could use TestRc to test if the previous test worked or not.

AddToFunc MyFunction
+ I Test (x picom) DoStuff
+ I TestRc (NoMatch) DoOtherStuff

I got it to work with None (Picom), but (!x picom) is better. Thanks.

None (Picom) and Test (!x picom) do different things, so use the one that does what you want. The first, None (Picom) tests if no window is running with that name/class/resource, while the second tests if that binary exists.