Possible causes for illegal lineups

1.) Player being used more than once
2.) certain positions being used more than once - make sure that you have left tackle and right tackle instead of both as right tackle, etc. On offense, only WR left and WR right are allowed 2 players at those positions, all others can have only one. On defense, CB allows for 2 players at each side and middle LB allows for 4 players.
3.) Injured player in lineup, only possible if you set lineup before injury happened.
4.) Sold player in lineup, only possible if you set lineup before player was sold.