The specified period & is not defined

What causes this issue?

<GL:date_type>Date type</> &V1& is not allowed.
INCLUDE 'NA_NO_PROCESSING_1' OBJECT DOKU ID TX LANGUAGE E
INCLUDE 'NA_CHECK_ENTRY' OBJECT DOKU ID TX LANGUAGE E
DEFINE &FELDNAME& = '<LS>Period</>'
INCLUDE 'NA_POSSIBLE_ENTRY' OBJECT DOKU ID TX LANGUAGE E

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