The Invalid Send Bulk Messages error is issued in the following situations:
'MSISDN Attribute' must be of 'Phone Number' data type in <Send Bulk Messages>: you have a Send Bulk Messages element, in an action flow, that is not correctly filled-in.
Change the MSISDN Attribute property of this element to an attribute whose type is Phone Number.
Clicking the symbol will take you directly to the widget mismatched property.
Send Bulk Messages | About Error Messages