Please do as follows to resolve this error message:
Go to the Leave module, click Leave details, and check your employee's category and section assigned.
Click Setup, Leave Policy.
Select your policy to check how Annual Leave has been assigned. You may have a selective category for each Annual Allocation.
Ensure that your employee's category is assigned in this leave policy for the Annual Allocation.
If it is not assigned, click the plus button in the centre to add the employee's category.
Then select the Annual Leave Type ID, Annual Allocation, and Costing Method.
Click Save.