SQL_Error_Diagram

Issue:-

Microsoft SQL Server Management Studio

Cannot execute as the database principal because the principal "dbo" does not exist, this type of principal

cannot be impersonated, or you do not have permission. (Microsoft SQL Server, Error: 15517) 

Solution:-

A) use [DatabaseName]
    EXEC sp_changedbowner 'sa'

OR

B) In UI -- Database >> Properties >> Files >> Owner - change to sa