Excel Data Validation List Error

Artemio Valdes 

I had the very same issue. A validation formula previously working smoothly returned that alert once a new argument was added.

After a while I realized that if the cell you are checking contains already the value you are entering as filter for your validation list you get that alert. I solved deleting the relevant value from the cell before adding the new argument in the validation formula. New validation formula was then accepted. When writing the value again in the cell the validation was working perfectly. 

Not sure I've been clear enough so I will write an example. My formula was something like:

=IF(AND(A1<>"RS",A1<>"GA",A1<>"GG",A1<>"YM", B1<>"I",B1<>"CS",C1<>"PREC"),KPI_List)

You can see that I was checking the values of various cells at the same time. I had to add another check (adding it at the beginning of the AND function below):

=IF(AND(A1<>"MJ", A1<>"RS",A1<>"GA",A1<>"GG",A1<>"YM", B1<>"I",B1<>"CS",C1<>"PREC"),KPI_List)

This returned the alert you are describing. The value "MJ" was in cell A1. I deleted, "MJ" from the cell A1 and tried again to modify the formula. This time it worked and when writing back "MJ" in cell A1 the validation was correctly working. Strange behavior.