Can a creditor sue you if you are enrolled in a debt settlement program?

Yes. A debt repayment program other than a chapter 13 bankruptcy, does not confer legally binding terms on creditors to prevent them from seeking litigation. AN exception obviously would be if the creditor has signed an agreement agreeing not to file a lawsuit as long as the debt repayment obligation is met. It is however, very doubtful a creditor would agree to such.