I think rh-php71 is not installed, see Collabora uses rh-php71 without requiring it - #2 by robb. Please install it and retry Nextcloud configuration:
yum install rh-php71
signal-event nethserver-nextcloud-update
Does the “nextcloud” database exist? Following command shows the databases:
mysql -e "show databases"
Are there errors in /var/lib/nethserver/nextcloud/nextcloud.log* ?