I assume this is MODX Evolution. Something on your site is trying to load the docmanager document class file and it's not there.
IIRC, docmanager was removed from Evolution because of the problems it caused, but I could be thinking of something else.
I found it here:
http://modx.com/extras/package/docmanager?version=4d556ac7b2b083396d0001fa
Thank you, BobRay. Yes, it is MODX Evolution. So if I downloaded that file and put it in /manager/includes/, would it fix this issue or cause problems since it was removed from Evolution?
I honestly don't know. Your two options are to try installing docmanager, or remove whatever is that's calling it. If you scroll that error message to the right, you should see a clue to what file is trying to access it.
Unless you're using docmanager for something critical, I'd recommend trying to remove or disable it, but someone more familiar with Evo might give you better advice.