It happens when you are trying to delete a SQL user. To change the DB_Owner to some another “Database Principal”; simply Drill Down to your Database in Sql Server Management Studio and further more Drill Down to your_DB_Name >Security > Schemas > db_owner > right click > select properties. You would find the name of the “Database Principal” that you want to delete. Change this to some another “Database Principal”; for Example, to “dbo”.
You need to make the same changes to all the schemas where the “Database Principal” is having/owning the role. For Example, if it is having “db_owner, db_datareader, db_datawriter” roles; then you have to make the above said changes for all these 3 schemas.