If you are getting an error message saying "The database is in single user mode" then please use SQL Management Studio to verify the following:
1. Connect to the target SQL server (relational)
2. Open the "Databases" folder
3. Open the "System Databases" folder.
4. Make sure the "model" database is not set to Single User. The "model" database is the template for all new databases so if it is set to use "Single User" then all new database will also be set to "Single User".
To verify, right click on the "model" database then select Properties. Select the Option page then make sure it is set to "MULTI_USER"
5. For the database already created, you can do the same by locating those database, it should be labeled as "(Single User)" similar to screenshot below, just follow the same step above to change its property to "MULTI_USER"