well, your errors relate to what you're asking in the first command, - set db to single user mode. if there is at least one more connection (which can be even yours0 the command will fail. you can write a simple script that would create a script that kills all connections to that database. fire that script before running sqlmaint and set a job step dependency "on success" with sqlmaint step.
Just to be clear: ms_sql_dba is right; your job is failing because your database is not in single user mode. SQL backups can run with multiple users logged in, but in your case, the first task in your job is to "repair" the database.
I'm guessing that you are using the maintenance wizard; if so, skip the Integrity page on the maintenance wizard (make sure nothing is checked).