Can't call the Moon Shot after I killed the final boss?

As BenBrocka pointed out, quit your game (ie exit out of the menu), and load your game again. You'll continue the game from the same area as the fast travel before the Warrior fight.

Don't join a friend's game or anything, otherwise you have to do the whole Hero's Path fight against Loaders again.

All credits go to BenBrocka

Also, additional tips from the gameFaqs forums:

Happened to me, I jumped in the lava to die, and then fast traveled to pass. ran back through and his body was in the right spot and I could hit the button. It DID glitch out hilariously though and fake Jacks kept appearing and attacking, and running through the cutscene, like when lilith said "no rest for the wicked" and one ran right in front of the screen, I almost died laughing