There are 3 probable reasons:
Your bank has not yet authorised 3D Secure 2 authentication*
On your order, the credit card security check failed
The payment exceeds your credit card limit
In all cases, please contact your bank for further assistance.
Please note: For all credit card payments, data is exchanged using the 3D Secure 2 authentication procedure, to comply with the *PSD2 (Payment Services Directive 2) payment regulation compulsory in Germany.