At the Dino Training Card Mission set, at the first card, I have to Assist a T-rex.
I killed several Human with red circles, in the right-upper it said: Marnix > AssistBite > [OtherPlayer]
So that sign with assist kill.
But the objective doesn't update. I still need an Assist T-rex.
What to do?
Send a ticket to 1:1 Support and report the problem. (Project Blackout -> Let Us Know -> Bug Report - Game)
You can try switching to a different card in your Dino Training Mission Card Set. Upon completion of those missions, try going back to the uncooperative card and see if the T-Rex assist works.
Otherwise do as GardenOfEden says and submit a 1:1 Support Ticket.