To progress the brotherhood storyline i got this quest. I started doing it and then i reached the HQ where i killed everyone as was the goal exept for deacon who i cant target in vats and who's health doesnt drop. I need to kill him to complete the quest.

I tried fast traveling out of there and coming back, tried starting the quest without power armor cause it seems to cause a lot of issues overall and nothing changed. I even went in to the HQ prematurely and killed everyone before even starting the quest hoping that the immortality trigger wouldnt apply pre-quest. But then when i got to one of my settlements where Deacon was he started shooting everyone and was still immortal.

Im playing on ps4 so i cant just use consolecodes to bypass this stage like everyone i found online doing.