This database does not have a valid owner, so you cannot install the database relational diagram support object.

After creating a new database or attaching a database, if you want to add a relational table, the following error occurs:
The database does not have a valid owner, so you cannot install the database relational diagram support object. To continue, use the "file" page or ALTER in the "database properties" dialog box
The AUTHORIZATION statement sets the database owner as a valid login name, and then adds the database relational diagram support object.
The first method does not work, and the file owner is sa. The user provided a way to run the following command:
Alter authorization on database: mydbname TO sa
Modify mydbname to the actual database name, and set the owner to sa.
There is another solution:
1. Set the compatibility level to 90 (2005 to 90) (2000 to 80)
USE [master]
GO
EXEC dbo. sp_dbcmptlevel @ dbname = 'database name', @ new_cmptlevel = 90
GO
Alternatively, right-click the database you restored, choose Properties> Options> Compatibility Level, select sqlserver2005 (90), and click OK.

When you expand the "database relationship diagram" node under the database, you will be prompted, "This database lacks one or more support objects required to use the database relationship diagram,

Whether to create ", select" yes.
2. If the problem persists after performing the above operations, continue as follows:
Select your database, select "security"-> "user", and select dbo to open the attribute page. If the logon name is empty, create a new query and
Use [your database name]
EXEC sp_changedbowner 'sa'
After the execution is successful, select the "database relationship diagram" node and prompt "this database lacks one or more support objects required to use the database relationship diagram, whether to create ",

Select "yes.