Let's explain it a bit as well ...
column_name
Is an explicit column reference. Any reference to the table being modified must be correctly qualified by either the INSERTED or the DELETED prefix as appropriate, for example: INSERTED.column_name.
As you can read: you can only specify column-names if, and only if, they are qualified by INSRETED ( the new values ) or DELETED ( the old values ).
"Walking on water and developing software from a specification are easy if both are frozen." -- Edward V. Berard, "Life-Cycle Approaches"