Microsoft Access groups on every control in a group level that was created with the Promote button. The control you added ...

Microsoft Access groups on every control in a group level that was created with the Promote button. The control you added to the group header or footer for this group level is bound to a field of one of the following data type that can't be grouped: in a Microsoft Access database (.mdb), a Memo or OLE Object data type; or, in a Microsoft Access project (.adp), a binary, varbinary, or text data type.

Recreate the group level again by using Group By Table or add the control to a lower group level.