This can happen when a player who was set on lines or pairs is no longer available to play. Reasons could be that we was scratched, released, sent to the minors, or is injured. This created an empty spot on the line or pair. If you click on the "View All Lines" or "View All Pairs" link, you can easily spot the line or pair with the empty spot. Simply fill all the empty spots with different players and the caution message will go away. If you do not fill the empty spots by the start of your next game, your coach will fill the spots for you.