Im creating a master-detail field on a custom object I just created and I was wondering why this standard object (AccountContactRelation) does not appear when I want to select it as the Master... Thx and sorry for my english in advance
AccountContactRelation standard object cannot be in Master-Detail relation. Same goes for any other junction object in Salesforce.
A junction object can't be on the master side of another master-detail relationship.
More to read about relations in SF: https://help.salesforce.com/articleView?id=sf.relationships_considerations.htm&type=5