Upgrade nextcloud 17.x.x to 18.0.1 from the terminal when the rest fails.
I’m assuming you have nextcloud in /var/www/nextcloud it may also be found in /var/www/html/nextcloud
sudo su
mkdir /var/www/nextcloudold
mv /var/www/nextcloud/* /var/www/nextcloudold/
cd /tmp
wget https://download.nextcloud.com/server/releases/nextcloud-18.0.1.zip
unzip nextcloud-18.0.1.zip
cp -r nextcloud/* /var/www/nextcloud/
cp /var/www/nextcloudold/config/config.php /var/www/nextcloud/config/
NOTE: If your data folder is in the nextcloud folder, you must move it with this step, if you have it outside the nextcloud folder (which is best) you do not need to move it and skip this line:
mv /var/www/nextcloudold/data /var/www/nextcloud/data
cd /var/www/nextcloud/
sudo -u web1 php occ upgrade
rm -r /var/www/nextcloudold
sudo -u web1 php occ db:add-missing-indices