Hello Asad,
for me it looks like you have not maintained a payment format for the house bank.
Please check, that you have maintained in the tab "Payment Formats" of the My Bank UI a format, that support Direct Debit. If you e.g. select "SEPA Direct Debit", then you can select for the bank account the payment method "Sepa Direct Debit". When you choose the German Format "DE DTAUS" (then the method "Direct Debit" and "Direct Debit (Authorization for Payer's Bank)" come up.
So if you select the correct format, then you will be able to select direct debit as payment method.
Best regards.
Marlene Katzschner