The write access check error tells you what the issue is. Your webserver needs write access to the moodle/blocks folder in order to be able to place the plugin there.
The instructions for that may depend on your server and the web-server user (www-data on Apache), but is likely to be something like:
sudo chown -R www-data:www-data /var/www/html/moodle/blocks
sudo chmod -R 770 /var/www/html/moodle/blocks