This is a common error when the user performing sequence studies is NOT the same one that is creating the balance.
If this error shows up, check the tool tip in the order sheet to double check by hovering your curson over the top right of the red cell. Note the Model for the orders that are turning RED.
if both of these are true, open the Line Balance used to create the study and double check the models that are in the study using Edit>>Routing & Model Details. If the Models in question dont exist try refreshing the scenario using Edit>>Refresh Scenario Data.
If that STILL doesnt work, go back into the routing that was used to create the Line Balance and be sure to add the Models that are missing to at least ONE activity within the routing. Then refresh the Line Balance and the Sequence Planner study in order.