In between the time that you dragged a population to the compensation window and when you selected "Calculate" in that window, one of the populations that you dragged has been deleted! Now, FlowJo can no longer calculate the compensation matrix.

Determine which population has been deleted (you can click on each box to find the name of the population that you dragged into that box). With that box selected, hit the "delete" key to remove it, and drag in a new population that is appropriate for the box.

You can get more information about compensation in general, which has a tutorial showing you how to compensate in FlowJo, or more information specifically about the compensation window.