when i go to the credit page and select my tf2 inventory, it says i don't have anything in it even though right next to the "team fortress 2" inventory option it displays the number of items in my backpack. how am i supposed to get key credits if this site thinks that i don't have a backpack?

We are not TF2 warehouse where all items could be used as credit. The credit page only displays what could be deposited as credit. (i.e. keys).

You can deposit other kind of items through your shop page https://dispenser.tf/myshop and put them to market to sell to other users for credit.