Statutory leave doesn't apply to workers who fall under IR35 working rules. These are deemed employees.

Trying to add a statutory absence to a deemed employee will give you the following warning:

If you're unsure whether you have a deemed employee, check with HMRC at gov.uk.

If you're certain you don't have any deemed employees, but still see the above message, you need to correct this. For more information, read Off-payroll working rules - IR35.