Please, please help! I just did the same thing. Put the install directory where it's supposed to go, activated plugin from my dashboard, and now my whole site is down with the same error and I cannot access my admin panel at all.
This is totally paralyzing to my business until it's back up. Please let me know what to do ASAP. Thank you.
Update: I deleted the plugin from the directory and the site came back up, but with some of the functionality messed up. I'm not sure how to fix it. Please be in touch with me to figure out how to uninstall properly.