Operation mode & is not a production operation mode.

What causes this issue?

The work process allocation of productive instances must only be
assigned to "productive" type operation modes.

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?

Specify the name of a productive operation mode, or use F4 to display
the list of productive operation modes and choose one of them.

Error message extract from SAP system. Copyright SAP SE.