You are already in the middle of executing another bit of code. Go the
the VBE (Alt-F11), and stop any code from executing by clicking on the
blue square.

Most likely you entered this situation because your code reported an
error, and you selected "Debug". So the VBE is waiting for a response
from you - to either stop the code, or progress through the code with F8
(progress line by line) or "Play" (right pointing arrow - start code, or
continue executing code).