Thanks Kamal, that's exactly what I needed, and is a bit safer than setting the whole Wordpress install dir to 777.
sudo adduser <username> www-data
sudo chown -R www-data:www-data /var/www
sudo chmod -R g+rw /var/www
its worked for me
Ok, After reading some article,I think I find the right way to solve this problem.
Add this to wp-config.php: define('FS_METHOD','direct');
Don't use any order, you should change the permission of /yourwww/wp-content and /yourwww/wp-content/plugins to 777. Of course ,the /yourwww/wp-content and the /yourwww/wp-content/plugins should have the 777 permission. The problem is about permission. After you do this,you will never face this problem again.
This is user's permission problem that will be solved by method given below that worked for me.
step-1 : First open wp-config.php file of your wordpress installation folder.
step-2: Copy below code and paste it at the end of the wp-config.php file.
step-3: Now open your droplet access console and enter root as username and its password.
step-4: Copy below code and paste it into access console where you have to specify your plugin directory path and then hit enter.
chmod 777 /yourwebsitename/public/wp-content/plugins/
Note: Replace your website name in the place of yourwebsitename.