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.
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
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.