This question already has an answer here:
I would phrase this sentence as "de ne pas pouvoir" or "de pas pouvoir", but I have never seen the "de ne pouvoir" construction like this. In what circumstances can you get away with only using "ne"?
« Elle a regretté de ne pouvoir faire ses adieux. »
Thank you for the link. Unfortunately, my French is not too advanced yet to capture the minute details of the answers there written in French.