Oh, I see luci-proto-vxlan requires 22.03.0 while vxlan has a minimum of 18.06.0. Your Spitz AX is advertised as 21.02 (GL GUI → System → Upgrade).
Technically I suppose you could still do it without the LuCI interface… or hold tight until a future GL firmware drops. The next series are going to be based on OWRT 22.03.4, IIRC.