Hi, the NPC in your screenshot is a Move Tutor for Volt Switch. Have you tried teaching Volt Switch via TM? As per the TMs/Move Tutors guide, you can pick up the Volt Switch TM at the Mauville Game Corner. Let me know if that works for your Forretress.
Yes it works. I wasn't aware that fortress couldn't learn volt switch from a tutor. I didn't know it is supposed to learn it from a TM only. It worked, thanks.