BDC processing mode must be A, N or E

What causes this issue?

A, N, or E are the only entries possible for processing modes.
'A' stands for online processing, 'N' for background processing and
'E' for background processing unless errors occur.

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

System Response

How to fix this error?

Enter A, N or E.

Error message extract from SAP system. Copyright SAP SE.