Every time I try to give Virgil the serum he asked for, it doesn't give me the option even though I have it. It shows the “A” option but no text beside
A Loose End - Fallout 4 Guide - IGN
Cure for Virgil (Rocky Cave) - Fallout 4 Game Guide & Walkthrough | gamepressure.com
Fallout 4 - Bring the Serum to Virgil (or not) - YouTube