This could be due to selecting the wrong verification type for the selected sick reason. To enable the Return To Work form for a sick reason:
Go to Me tab then select Planner.
Add a new sick leave then click on Reason dropdown list.
Select +(Manage This List) then the relevant sick reason.
Tick Return To Work Form Required then click Save.
Click Close.
📌Note: After updating the verification type for the sick reason, you need to update the sick entry by resaving it on the employee's planner.