Hello @tourette95,
This error indicates that BEst is probably not installed.
Could you check if .brainstorm/brainentropy exists?
Or you may execute the steps proposed earlier by @Francois here, or alternatively the steps given here.
This should install BEst and allow you to then run the process "Compute sources: BEst" after a drag-drop in the Process1 tab (as illustrated earlier here).