This happens when you either miss putting in all the pink complexity boxes on your evaluation form or enter the info incorrectly. These are highly coded boxes, so even an extra space may cause an issue.
To fix, select the evaluation in the list in Charges & Documentation right-click Open Unsigned Clone. Take out the info in the pink boxes and then double-click to put the selections back in. Make sure you have a valid CPT code result at the bottom before you sign, save, and close. Then you will want to delete the original form, so the flow sheet does not see it. Lastly, refresh your flowsheet and try marking the function again.