Nextcloud is a free and open-source software, used for cloud storage and collaboration tools. Piktures is NOT associated with Nextcloud.
Nextcloud is a solution allowing you to build your own personal cloud server.
Piktures is able to manage, view, upload to, and download photos from your own instance of Nextcloud, thanks to the Webdav open protocol.
You should follow the instructions provided by Nextcloud to enable WebDav access