There aren't a lot of examples of Kasa's struggles with route running committed to video, but this is a decent one.
While we can never know who is at fault for the miscommunication on this play, Kasa's reaction after the fact may offer a hint. It appears he is supposed to be running a quick hitch at the sticks on third down, but he rounds off his route—as if to run a lazy out—rather than flipping his hips and squaring his shoulders.
The QB throws the ball where Kasa should have been, resulting in an incomplete pass and fourth down.
The next play on the reel, however, shows some potential for route-running skills, stopping on the cross at the hashmarks, then finishing out to the numbers where there's room between the LB and CB.
He breaks two high tackles, then rumbles on down the field for a big gain.
That play is exactly what we want to see from Kasa. If he can develop his separation skills, he'll be a nice weapon.