How do i treat erythrasma on the scrotum topically? do i use antibacterial soap or just use triple antibiotic ointment?

Make sure the diagnosis is correct. Several other problems are more common causes of redness, irritation, etc of the scrotum, and the treatments are different. See a doctor, ideally a dermatologist, for confirmation of the diagnosis if you haven't done so. If erythrasma is confirmed, topical clindamycin or erythromycin usually is recommended; triple antibiotic ointment may be less effective.

You can use both. Wash with soap and apply ointment. But make sure it is the right diagnosis for antifungals are used for that. Best to see a Urologist.