The column & is unknown

What causes this issue?

A column is not known because:
it does not exist
it is not active
an alias name was assigned for this column in the selection list

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?

You can perform the following actions:
Correct the column name.
Activate the base table if the column does not exist as an active
version.
If the alias name of a column was used in an association definition, you
must place the $PROJECTION addition in front of it.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.