At the official start of a matchday (usually Friday 20:30) your current account balance on the transfer market (manager mode) or in the lineup (challenge mode) must not be negative, otherwise you will NOT receive any POINTS for this matchday. Important: It does NOT count the time when the referee blows the whistle, which can be 20:32, but the official time. After the start of the first game of a matchday, you cannot change your lineup for the remaining games. All your lineup changes will only affect the next matchday one week later.
You should always have 11 players in your lineup, because for each unoccupied position you will be deducted 100 points.