If you already have a lookup relationship to an object, and that you don't have values populated on that field for records, then you cannot change the field to be a Master-Detail relationship.
Refer to the help documentation
You can convert a lookup relationship to a master-detail relationship, but only if the lookup field in all records contains a value.
and this knowledge article for details.
Salesforce will not accommodate a master detail relationship to a custom object that already contains data.
This is because creating a master detail relationship adds a new required field to the custom object (known as a "foreign key" to database administrators). Each data record in the object needs to have a valid value in the field -- it cannot be left blank.