Question : sql server add valid owner

TITLE: Microsoft SQL Server Management Studio
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Database diagram support objects cannot be installed because this database does not have a valid owner.  To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.

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Answer : sql server add valid owner

try this

alter authorization on database::databasename to sa

obviously change databasename to your database name