Are you trying to log in your Pi via SSH as ‘root’ or as user ‘pi’? Generally you shouldn’t have to set a root passwd (like Shoothere above said: use ‘sudo’), but if you did it shouldn’t generally be a problem. But how that is w/ Retropie and the Debian distribution on which it is based might be changed recently (I wouln’t know why, but hey).
Anyway, I would try to log in w/ the following command: ssh firstname.lastname@example.org.XXX
where 192.168.0.XXX is the IP address of your Pi on your (local area) network (see your router for said IP address). Could you copy/paste the error message of the command above?
2.a) quit into the command prompt on the pi.
b) Type cd .. (type this a few times until your are at the root)
c) Type clear
d) Type cd /etc/ssh/
e) Type clear
f) Type sudo nano sshd_config
f) the cursor will be at the top, use the curor keys and then locate and change PermitRootLogin without-password. TO PermitRootLogin yes. then hold ctrl and O. the ctrl X.
g) sudo passwd root. (Enter new password. Type exit)
The set of commands I pasted from my own written instructions i keep on my PC. The clear command was there so that the screen would be clean and the cursor would be at the top of the screen ready for editing sshd_config.
Hope these instructions help people 🙂