Hey there, Alan
When reading your question, it could be that you have an Entra free subscription attached to your billing account. If this is the case, then you are unable to remove the payment method or detach it. This would result in this sort of error appearing. To delete it, you must delete the tenant.
You can speak to support by opening a support request with the billing team (which is free to do) by going to Help + support
You will then want to create a new support request
You will then need to enter a brief description of your problem. I would use 'remove payment method' as a description > Then select the billing > Then select the subscription.
You will then need to scroll down to the bottom where it says, 'Need more help?'
Then click onto contact support.
Then click onto 'Create a support request'
This will allow you to open a support ticket with someone from the billing team who can look into what is happening. As I mentioned above, it looks as though you have an Entra free subscription that is tied to your account, which could explain why you are unable to remove the payment method from your account. However, the support team will be able to tell you exactly what is causing it and what to do.
Hope this helps,
Nathan