You cannot generate a round.
Cause: Almost always the player count. Cream of the Crop needs a multiple of 4 with at least 16 players, or exactly 25. Counts like 12, 18, 22 and 26 cannot be split into valid groups.
Fix: Bring the count to a valid number — add a player, or remove someone from the active pool in the group editor. See Player counts for Cream of the Crop.