Employees are not able to add or edit their own sickness by default as there may be set reasons admins want to put or to require an RTW form which employees might not be aware of if logging their own sickness.
Admins and managers have the ability to record and edit a sick day on an employee's record, however employee's cannot add or edit sickness to their own record. Once communication has taken place with the employee and the manager/HR, this can be added to their record.
Please refer to the following articles: