At a guess, you don't have permission to the /dev/ttyUSB0 device. Assuming you're on Ubuntu (or probably Debian or others) if you do
ls -l /dev/ttyUSB0
you should see something like:
crw-rw---- 1 root dialout 188, 0 Dec 9 21:25 /dev/ttyUSB0
(note that you will likely have to have the dock plugged in for this device to exist!)
Feel free to skip this next bit if you already know Unix permissions!
Here's a quick explanation of this if you don't know what it means. The first bit (crw etc.) is the permissions on the device. The c at the beginning indicates that this is a special 'file' type connected to a device. (Don't worry about that bit.) The next sets of characters show what the permissions to this device are: r for read, w for write, and x for executing as a program. These are in three sections, one for the owner of the device, one for the group of users that own the device, and one for everyone else. In this case, rw-rw---- means that the owner has read and write permission (but not execute), the group does as well, and everyone else has nothing at all.
After the permissions are the name of the owner and the group that owns the device. In this case, 'root' owns it and the 'dialout' group is the group. Since you are neither root nor in the 'dialout' group by default, you fall into that other category with no permission to access the device.
Now the solution!
There are a few ways you can deal with this. One is to use 'sudo' when launching screen:
sudo screen /dev/ttyUSB0 115200
This will execute screen as the root user, meaning you are now the 'owner' of the file. If you're okay with doing this every time, you're all set.
Another thing you could do is to make yourself a member of the 'dialout' group:
sudo usermod -a -G dialout [your user name]
This will add you to the dialout group and give you permission. You will need to log out and back in first before this will work though.
