Just finished the campaign on pc and did the option for the p90 and when I exited out of the game, No p90. Is this a common glitch or was it possibly something I did?
I think you have to watch the entire credits to get the prize.
Did you rub your belly and pat your head while standing on one foot and trying to touch your nose with your tongue? If not, try again, and do that while watching the end credits. Be sure to record your process and post it here… to help others 
I’ll be sure to do that!
Thanks, Thats exactly what happened most likely