It sounds like your tmp directory is not set up correctly to allow write access for everyone. Either that or the /tmp/uscreen directory is already created and your user doesn't have access to it. To check the perms for your tmp directory execute the following and look for the line for /tmp.
The owner and group should be root. The permissions on the left are owner, group, and others and can include read (r), write (w), and execute (x). For /tmp they should be all filled up.
If you don't have permissions to the tmp directory you can fix it as root
4 = read
2 = write
1 = execute
4+2+1 = 7
I don't really understand why screen isn't using a hidden directory in your home directory. Mine uses .screen.