Set your storage type to Webdav, fill in your nextcloud server address and port, set path on server to remote.php/webdav/backup-folder-you-want-to-use (NOTE!!! - NO pre or post /) for this backup set. Each backup MUST have a unique folder. Nextcloud user credentials each have their own tree, so as long as you have unique users for your backups, each one could be at remote.php/webdav/duplicati or something, but if you use the same nextcloud credentials for 2 backups, make SURE you have different