XDr ? Are you making the switch to 1x? or going for more gear range? - bucky rides
SRAM GX XD Cassette onto XDR freehub: I slid the Cassette onto the freehub, the splines of the Cassette and the freehub seem to engage, but the cassette doesn't slide on completely.
SRAM XD/XDR Freehub Installation – Wahoo Fitness Support
SRAM explains XDR freehub body, which will make 12-speed road possible - Bikerumor
Bike Freehub & Cassette Compatibility Manual
SPN Precision 12 Speed XDR Driver - SRAM AXS - Spin On These