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.
Double-click on the error line to take you directly to the invalid property.