in [[, empty strings are falsy, so this also works:

[[ ! $VARIABLE1 && ! $VARIABLE2 ]] && echo "OK"

The variables need a dollar sign: $VARIABLE1

help test shows what -n and -z do.

Never use a && b || c. It is not the same as if a; then b; else c; fi: when a succeeds but b fails, it will run both b and c.

I would not bother with [ unless you absolutely need compatibility with non-bash shells.

Try opening a terminal an typing echo 'Hello World!' My two cents: You can forget about Linux for a while. Using a terminal is more important. Here’s a classic guide: https://mywiki.wooledge.org/BashGuide

Cool operating system bro. Does it run KDE?