Why cant I buy the Cromwell B?

I had 20 rounds to play with it to try i t out. Now i wanted to buy it but the „buy“ button is not clickable? Somebody know why?

I thought that if they give me the tank to try i would get the chance to buy it aswell. Wouldnt that make sense?

I had 20 rounds to play with it to try i t out. Now i wanted to buy it but the „buy“ button is not clickable? Somebody know why?

I thought that if they give me the tank to try i would get the chance to buy it aswell. Wouldnt that make sense?