First, you should ensure that your username is included in www-data group. If not, you can add your username as www-data group

sudo adduser $USER www-data

After that, you should change the ownership of /var/www to your username

sudo chown $USER:www-data -R /var/www

Next step, you should change permission to 755, not recommend changing permission to 777 for security reason

sudo chmod 0755 -R /var/www
sudo chmod g+s -R /var/www

Hope it helps…