When installing a plugin off Moodle.org, it needs to be able to talk to your server. So it's a two way street ... your server searching/intereacting with the plugins ... then the other way once you've selected the plugin to install.
Assuming your server is Linux and that you have ssh access, login as root and change into moodlecode/blocks/
Then issue:
wget https://moodle.org/plugins/download.php/14134/block_course_contents_moodle33_2017060200.zip
(link might word wrap above - supposed to be all on one line)
Can ya get there that way?
If so, and the zip does download, you just have to unzip:
unzip block_course_contents_moodle33_2017060200.zip
That should create a 'course_contents' directory.
Change ownerships on the directory. chown apache:apache course_contents (assuming again that server is Linux ... your web server user might be www-data:www-data if Ubuntu.
Then, hit the moodle site with browser to finish install.
If you can't get there ... test vai command line:
curl -I https://moodle.org/plugins/
In the response to that curl command you might notice a reference to cloudflare-nginx ... which is the CDN in front of Moodle plugins server.
Also, on your end, the plugin check must detect and be able to write to moodlecode/somewhere
Are your files/directories in moodle code writable by owner?
'spirit of sharing', Ken