Check your Gateway settings to make sure it's setup for the currency you are trying to charge. Check the default currency for the Gateway and make sure it's correct Check your Chargent Order to make sure you have selected the correct currency.
If all of the above checks out and you are still having issues, contact your Gateway to make sure your account is setup to accept the currency you have listed.
Currency can be set at 3 different levels in Chargent.
1. Default currency (for all of Chargent)
2. On the Gateway Record (for all transactions being sent to that Gateway)
3. On the Chargent Order Record (for that individual record)
More information on currency configuration: