Completed a front/rear crossover replacement with the water pump and while I was running the engine coolant started shooting out!

Looks like I failed to replace this one tiny pipe, but now it won’t seat! It seems like the expansion fitting isn’t constricting enough on the nipple for it to seal and it’s just popping out. Any suggestions?

I’ve tried simultaneously pulling the fitting and the nipple, hoping it’ll lock into place but no luck.

I’ll probably order a replacement from Atlantic British but I’m hoping someone has a trick