I am following some tutorials and discussion on forcing uniqueness in Google Sheet Column (I need it for ID indexing). I have been using Data Validation for some time, but i have a problem with custom formula and using some functions. I know that that i should use
=COUNTIF($A:$A,"="&A1) < 2
But if I enter it to custom formula input box I am pressing "save" button and nothing is happening. It seems like google is stopping me from entering this formula. For other formulas I can save the validation and it is working. Problem is that I don't get any feedback(error) from Google, what is wrong in my formula.