I have seen this discussed before but have tried several formulas and have
not been able to do what I want to do. It is theoretically very simple: I
want to use conditional formatting on an entire worksheet, and for every cell
that is NOT locked, I want it to be shaded/filled with a color. I also have
values in my worksheet that are errors, and I read that conditional
formatting will not work with cells that have errors. How do I get around
this?

I tried:

=NOT(CELL("protect"))
=IF(CELL("protect") = "0",TRUE,FALSE)
=CELL("protect")

All with the 'applies to' field set to =$A$1:$I$35 (this is the effective
size of my worksheet) and the 'Stop if True' box unchecked.