Ndless is currently not compatible with OS 3.9. OS 3.9 has (once again) a downgrade protection that blocks downgrade to OS 3.6, do not upgrade to OS 3.9.
Unfortunately nothing can be announced for the moment about any upcoming Ndless update. Just stick to OS 3.6.
New users who just bought a TI-Nspire with OS 3.9 pre-installed will have to wait a bit for a solution (and there’s have always been one since 2010). Sorry for the delay, you can subscribe to this blog to be notified.