The OP is interested in the B and J seats, so the lack of DEF in row 31 may not be an issue here.

Looking at the second seat map which is the one QR displays to select seats from (rather than the first one which is more more accurate to the actual layout), it looks like those B and J seats are blocked or already selected. It is possible that other passengers may have already selected them thinking (incorrectly) from that layout that those were the seats in that row with the extra legroom.

Either way, neither of those seats are available for other passengers to reserve currently, and the OP may need to make a different selection.