To calculate summaries from child objects of a standard object, an app builder should evaluate two considerations:
An app builder is unable to change a look up to a master-detail relationship. A lookup relationship can only be converted to a master-detail relationship if the lookup field in all records contains a value. If some records have null values, then the conversion will fail.
A value is required in all records of the lookup field prior to converting to a master-detail relationship. As mentioned above, a lookup field cannot have any null values before it can be converted to a master-detail relationship. Users will need to populate the lookup field with valid values for all records before attempting the conversion.