Custom Board Positions in Tournaments

I am trying to let students make custom board for an Arena Tournament but I do not understand what boards I am allowed to use. Lichess let me use this position with all the Knight, but did not allow the position with all the pawns. Knight FEN code: nnnnknnn/nnnnnnnn/8/8/8/8/NNNNNNNN/NNNNKNNN w - - 0 1 Pawn FEN code: rnbqkbnr/pppppppp/pppppppp/pppppppp/PPPPPPPP/PPPPPPPP/PPPPPPPP/RNBQKBNR w KQkq - 0 1   

You cannot use them because they are invalid positions. Positions that aren't invalid can be used.