Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
It seems like you still using 3.x version, you have to manually install it.
But I was unable to install through your Joomla backend, something is blocked.
Perhaps you can download my attached file and extract it put it on this file location JoomlaFolder/plugins/user/.., then try go to backend > extension > manage > discover > click discover button > check is there got `User - Easydiscuss` plugin > if yes, tick it and install it.