Diagnosis
You tried to create a tab title element that does not have the type"Pushbutton". Tab titles must be pushbuttons, because a function has tobe triggered when the user chooses one.

System Response
The tab title cannot be created.

Procedure
If you want to convert an existing element into a tab title, choose"Convert -> Pushbutton" to convert it to a pushbutton, then move itinto the title row of the tabstrip control.
If you want to create a new tab title, choose "Pushbutton" from theelement palette and place it in the title row of the tabstrip control.
If you want to use an ABAP Dictionary field as a tab title, first placeit outside the tabstrip control area, then convert it into a pushbuttonas described above before moving it into the title row of the tabstripcontrol.