Anti Tank Gun No Upgrade Button Showing

Can anybody explain to me why this is happening?

I have already research Anti Tank to Level 4

No upgrade button is showing up on my Anti Tank Gun lvl3.

I acquired this unit after I capped an urban province.

I did a restart to the Call of War App on steam. Even did a restart to my pc and internet. Tried it on mobile as well and on a browser. Still no upgrade button.

I'm playing on an Arm's Race Event Map.

Game ID#: 3742546