No, you can require them to use sick leave only. PTO is considered as vacation in terms of the Department of Labor compensation regulations. It is a benefit that they get paid for when their employment ends. Most employees would want to use their PTO so you require them to use the sick leave and it has to be in the policy and in the letter and allow them to use the PTO when the sick leave is over. If you only have PTO, they may use it but they have to let you know that they want to use the PTO.