Custom data validation with formula doesn't work & acts as if it was "any value"

Hi all,

first post, so greetings & thanks for having me here. Run into a problem when I tried to set up a custom data validation in one of my spreasheets (attaching sample).

"Week No" column is formula based, so values in it should be: =WEEKNUM(A2;13). Column A is where I have my dates. Now when I try to set up data validation, so it's the same as the formula in the cell, i.e. =WEEKNUM(A2;13) it seems not to work. I can click on a cell and type in anything I want to. Can't you help with this issue? I used this logic before on other sheets and it worked.

Thanks,
M.

Worked like a charm. Thank you!

One more thing: even if I have a data validation rule on a cell that's based on a formula I can still hit "delete" or "backspace" and erase the formula. Is there a way to prevent that from happening other than locking cells?