TalentLMS Plugin PHP error

I am just added the TalentLMS plug-in and added it to my domain. I set the setting so that it logging in stays within system and when I did so this error came up after I logged in:

Call to undefined method WP_Error::exists() in /home/content/a2pnexwpnas02_data01/27/3383327/html/wp-includes/capabilities.php on line 509.

I have people putting people into the system soon so I need this system to be fixed as soon as possible. Any help please?