Unable to add content or install plugins on Wordpress

Did you try to change the chmod via ssh?

First find out the username of the PHP/Apache process by creating the following PHP script:

<?php
$processUser = posix_getpwuid( posix_geteuid() );
print $processUser['name'];
?>

Access this script on your browser and it’ll print a name, usually www-data or apache. Then execute chown as follows:

chown -R username:username /var/www/html/

Replace username with what the script printed.

Heya,

The wp-content directory and its subdirectories need to be writable by the web server. This directory is where uploads, plugins, and themes are stored.

sudo chmod -R 775 /var/www/html/wp-content

Regards