If department input is set as mandatory in the
Document types register, but no department is specified on a document, a
warning is shown: Department must be specified.
The same warning is returned if a fixed asset
has Posting Method set to 1 -
Department and cost center from fixed asset header, but no department is
specified in the header.

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Solution
1. Specify a department in the document or fixed asset header.
2. Disable mandatory department input in the Document Types register or in
the Fixed Assets register.