You are too early. The booking window should be 6 months but it's often (always) less than that due to works mainly. Check regularly on nightjet.com or cd.cz -> always better to book through the railway company.

Btw if you're coming from Paris it's possible to take an earlier TGV to Offenburg or Freiburg and board the night train earlier too.

Okay, thank you for this! When I try to reserve just a normal seat instead of the sleeper / couchette it says  “all seats already fully booked” does this mean that I am still too early for these or do I need to look an alternative train? 

This is likely a (wrong) generic message. Where have you tried ? I'll double-check in case.

Thank you so much,I used DB Navigator. I thought that may be the case as you explained that I am too early. I wanted to try and get trains/accommodation booked for this leg of the journey as soon as I could to secure availability seeing that the Euros are being held in Germany at the same time! 

DB cannot book night trains (or even any mandatory reservation).

Tickets are on sale until 1st May at the moment through ÖBB/Nightjet.

My advice would be to check regularly then :)