Things that i tried were : 
1. Try executing the simple command fist and see if the out is comming.
  example : system('ls -l > output.txt') // This will output the file in public 
                                            //folder
2. After that i tried giving the permission 777 to the files and folder 
    that require and access and create a file.
    example : chmod -R 777 /var/www/html/PATH_TO_FOLDER/public
      
3. Search your php.ini with this commands: php --ini
  For Me: Loaded Configuration File: /etc/php/7.2/cli/php.ini
    In your php.ini file, set variables_order = "EGPCS".