Unable to make a purchase on Tank Force Game

HI

Although I have managed to make a coupe of purchases in the past, I am now unable to make any on the game Tank Force, all I get is the dreaded message below.

My iCloud payment and other payments I make are getting through, but not in the Tank Force game.

I even put some money onto my apple account, this has been happening for a few weeks now.

the Dev's in the game have told me to contact Apple, but that's a mission in itself, I have never managed to find the link, keep going round in circles!!

Any help would be much appreciated!