Reply To: Download and Install LineageOS 16 on Nexus 9 – flounder

I think the TWRP asks for the password because of encryption.
I will give you more data on this.

@cblx5

You can either flash another ROM or you can go back to stock Nexus 9 FW by flashing the stock images:
https://developers.google.com/android/images#volantis

You can follow the following guide: How to flash Nexus stock firmware