Move your Ingots to “Trader” and buy Credits.
Then move Credits that you already bought to the same place where you put Ingots.
Go to the “Production”.
Now you can buy Ingots with your Credits.
There is also an option to buy Ingots of every resources with Rebelium.
If you make mistake, there is an option to refund it back to “Trader”.
But sales are cheaper than buying. So keep it in mind, and think twice what you are buying.
After exchange don’t forget to move ingots to you cargo containers.