Hi
Master-detail relationship is like parent-child relationship where, master represents a parent and detail represents a child in which master object controls some behaviors of the detail object. Like whenever a Master object record is deleted then the detail object related to it also gets deleted.
So, we can’t create a Master-detail relationship on existing records. To do that, we have to create a lookup relationship for that record and then convert it into master detail.
To convert a lookup relationship to a master detail the existing record should consist of valid lookup field values.
Thanks.