Posting or simulation not possible. See long text

What causes this issue?

Posting area 1 has already been posted.
Posting is not supported for posting area 2 in accordance with the
posting rule.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

To post, change the posting rule by choosing the 'Other Attributes'
pushbutton.
If the bank statement item is complete, change the status by choosing:
Menu -> Process -> Change Status -> Completed.

Error message extract from SAP system. Copyright SAP SE.