I assume you have one single-seat license and you try to open FePEST, while FEFLOW is running in the background. Could you please check it? If my assumption is true I suggest to close FEFLOW if you want to use FePEST. Both FEFLOW and FePEST require a license.
If you want to post-process your model while FEFLOW is running, you can always use the FEFLOW Viewer. The Viewer does not need a license.