Cannot pay my bill through my Ameli account

Hello
My husband has received a notification that he has an outstanding balance to pay on his account but when he inputs the invoice reference the following message appears and he is unable to proceed 'This mailing reference is not open for online payment.'
He has also tried to pay via bank transfer but the bank flags that the name of the beneficiary doesn't match the account and prompts him to go through several security checks.
Has any one else had this problem? or know how to resolve it?