There are two possible reasons why you're unable to enter an SSP, alancowden55.
First, if the total absences of the employee is less than 3 days. The SSP will start on the fourth day from the day they were absent, for the first three days are considered as Waiting Days.
Lastly, it's possible that the employee have not reached the average weekly earnings of at least the Lower Earnings Limit (LEL) for National Insurance to qualify for SSP.
I have articles here that you can browse for the detailed information of the Statutory Sick Pay (SSP):