The CRM Validation Rule works Offline but not Online.
The Validation Rule contains ISNULL(TEXT(Picklist_Field__c)).
ISNULL determines if an expression is null (blank) and returns TRUE if it is. If it contains a value, this function returns FALSE.
Change the Validation Rule to use ISBLANK(TEXT(Picklist_Field__c)).
ISBLANK determines if an expression has a value and returns TRUE if it does not. If it contains a value, this function returns FALSE.
It is important to use ISBLANK instead of ISNULL in new formulas. ISBLANK has the same functionality as ISNULL, but also supports text fields.
SFDC Help Documentation: Formula Operators and Functions
CRM Help Documentation: Offline Validation Rules
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