There is no official debian package for the nextcloud client. There have been a handful of RFP bugs reported but it looks like no one has taken this on yet. I want to get more involved with debian packaging so this might be a great first package to maintain. For the time being, the owncloud client is still backwards compatible with nextcloud. Unfortunately, the version that ships with Debian stable (8, jessie at the time of writing) is a bit old. When I tried to connect to my nextcloud instance it complained that my password was incorrect. Luckily, there is a slightly newer version available in jessie-backports  which has no trouble connecting to nextcloud. The steps to get a working version of owncloud-client to work with the latest stable version of Nextcloud are as follows:

1. If you have not already, enable jessie-backports
   1. Open up /etc/apt/sources.list
   2. Append deb http://ftp.debian.org/debian jessie-backports mainto that file.
2. Run sudo apt-get update
3. Install the latest version of owncloud-client with sudo apt-get install -t jessie-backports owncloud-client

You should now be able to connect to nextcloud without any issues.