Interesting,
Looks like you had to to unlock all the cells first, and then lock the cells you want protected before you protect the sheet.

Yes, errors often occur when

Yes the first time it succeeds. But often I forgot to unprotect ActiveSheet before rerun the script
The second time when the sheet is protected, there should be an error when this code is run:

So I need:

I've heard advanced coders always say
it's good practice first to check before you code.
So "If a condition occurs do this; if not, do that."
should be done always to avoid "strange" errors.
It may prevent me from always asking questions.
I can work it out sometimes if it is practised.