I'm producing coins and I'm even taking them back to my headquarters, but for some reason they're not being carried to the barracks, and both the barracks and HQ have carriers available and idle,
and whenever I put soldiers into the barracks to train they just go out and say there isn't any coin,
am I missing a setting somewhere or does it require a higher level of carrier or something?

Try changing the carrier to a merchant and then set trading place between where you have the coins and the barracks.
You can then choose which goods they will trade by pressing set merchant.