1) Make sure the User Type assigned to your profile has not restricted access from you paying AR Balances. You may need to speak with your Head Administrator to check or change this option.
Go to Settings > User Types.
2) If you are using the Moneris gateway, the gateway may be having connectivity issues.
Go to Settings, Payment Gateways, edit, set it to inactive and Save.
Pay the invoice then go back to Settings, Payment Gateways, edit, set to active and Save.