No such modem: back

I need help with my login program or modem because when I launch the program it comes up with the error "startup:7: No such modem: back", however the code states that the modem is on the right, which it is:

local side = "right"
password_server = 210
rednet.open(side)

Thanks

Moved to Ask a Pro.

Well, your error message indicates line 7, but you've only posted 3 lines of code. Please post your full code (between code tags) so we can see what you're working with.

Code tags:

[.CODE]
code here
[./CODE]

Just remove the dots before CODE and /CODE and it'll look like this…

code here