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.