When position requirements fight each other

Three DTs, two DEs, three LBs and two CBs on a ten-slot board leaves exactly nothing spare. The solver will tell you when a board is over-constrained.

Positions are mutually exclusive, so each unmet position requirement claims a whole slot. Add them up: if the claimed slots exceed the empty ones, the board cannot be completed as written and no amount of buying will fix it.

Everything else layers on top. A city requirement, a rarity requirement or a special-set requirement is an attribute of a card that is already filling a position slot, which is why "one more Linebacker" and "one more Defense" can be the same purchase.

The solver builds the remaining slots for you with this in mind — position first, then it layers the other requirements onto compatible slots and tells you if one of them has nowhere to go. If you see that error, re-read the challenge panel in game; it is usually a miscounted requirement rather than a genuinely impossible board.