Sometimes you might get the above error message from SQL Server when you try to change the user mode from MULTI to SINGLE.
You have to explicitly kill the connection in order to solve the problem. First you need the dbid of your database. Run the following command.
Once you get the dbid you can check the connections to the database. Run the following command.
spid is the connection process id. We have to kill all the spids in order to run our command which sets the MULTI USER mode. For example think you get the spid as 10. Run the following command to kill the connection and set the database user mode.