The journal header must be set for VAT Analysis if the journal contains VAT.
However, Line 1 of the journal can't contain VAT as this is the Control line.
This is why you can't add a VAT code on this line.
Note:
If the VAT value is positive it will be added to the Debit column to balance the journal.
If the VAT value is negative it will be added to the credit column to balance the journal.