I'm SO frustrated! I mocked up this design the other day with no issues and today none of the functions are working. I dragged my image in, placed it behind the letter I want to use as a mask, selected both, then got this message???
Being able to see your layers panel would clarify, but the message is pretty self-explanitory:
The selection cannot contain objects within different groups unless the entire group is selected.
You have objects from different groups selected. I assume that the entire word is a group but you only have one letter selected. Either select the goup, or ungroup and select the letter.