Field & is not defined as a search field in this line layout

What causes this issue?

The cursor is on a field which is not a search field in the current
line structure.

How to fix this error?

If you position the cursor on the command line and select F5, a window
appears which contains the fields which you can search for in the
current line structure.
You can only search for the content of other fields in the following
way:
select a line structure in which the field is defined as a search field.
your system administrator can define the field as a search field in the
current line structure in invoice verification via customizing.

Error message extract from SAP system. Copyright SAP SE.