Currently the validation error is a predefined message for the custom picklist Banner__c
.
I wrote the following validation error to create custom error message.
It seems to be an esay task, but nothing is woking in my case. Still the predefined error message is coming. I tries the following rule for this.
ISPICKVAL(Banner__c, "")
ISPICKVAL(Banner__c, "NULL")
ISBLANK(TEXT(Banner__c))
TEXT(Banner__c) = ""