A different bank is already indicated as being the opening bank

What causes this issue?

Only one bank per bank indicator can be indicated as an "opening bank".

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

System Response

The system sets the switch back.

How to fix this error?

If you want this bank to be the opening bank, you will have to change
the indicators for all the other banks so that they are no longer open.

Error message extract from SAP system. Copyright SAP SE.