'Has Variants' field is disabled when trying to add Item Variant on update

My client has a requirement to add variant for a product that already created. The decision was based on the product research team. When i tried to enable Has Variants checkbox to add Item attribute based variant, it was showing disabled. So, i tried to check in doctype field settings. There what i found is Has Variants was displaying depends on this condition : eval:!doc.variant_of. So, i checked variant_of field settings. There i found Read Only,Set Only Once properties enabled and Display Depends On condition variant_of. I’m a newcomer to ERPNext and couldn’t find the connection or whether it has anything to do with the disabled situation. I can perfectly add Item variants on insert time, but not on update.