When adding sickness, you will receive the error message 'Can't book after employee leaves on...' if they have an open sickness on a previous employment.
To resolve this, follow the steps below:
Search for the employee and go to their profile.
Click Switch Employment at the top of the page.
Select the employment under Previous Employments.
Go to Absence, then Sickness.
Click the sickness record with the status Open.
Enter a date in the Last Day of Sickness field.
Click Save.
Click Switch Employment and select the current employment you need to add sickness to.
Go to Absence, then Sickness.
Click Add Sickness. You will now be able to save this without any errors.