If you were able to add time worked in lieu for other employees but are having issues with a certain employee, check the start date of their employment. You will not be able to add time worked in lieu prior to their employment's start date.
To resolve this:
Search for the employee and go to their profile.
If they have several employments, click Switch Employment and select the relevant one.
Click Employment, then Position.
Look at the In post from date in the Post Details area. You will only be able to add time in lieu after this date.