Account alloc. & not allowed (Check your entry)

What causes this issue?

Account allocation &V1& has not been defined.

System Response

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?

A. Use defined account allocation (Possible entries F4)
or
B. Define account allocation using the Customizing functions.

Error message extract from SAP system. Copyright SAP SE.