If you are running your website on digitalocean and getting error while updating or installing new wordpress plugins and themes then this solution will help you to solve this problem.
I solved this issue by doing following two steps
1] Step 1
First login to your droplet using terminal. And run following command
sudo chown -R www-data:www-data /var/www
2] Step 2
After that you have to add following line of code inside wp-config.php
define('FS_METHOD', 'direct');
It will stop asking for FTP login details. And you can install or update existing plugin themes or wordpress easily.