Can't find "root joint" in path

Isaac Sim Version: 4.5.0-rc.36
Operating System: Windows 11
GPU information: Nvidia GeForce RTX4060 LapTop GPU
GPU version: 32.0.15.7602

All the files and . usd attached below.

https://drive.google.com/file/d/19CgFszKomdpHbNoOaYbiUC5aGf1I-P3g/view?usp=sharing

I’m trying to modify the tutorial file” cobotta_900” into my UR20 version, for RMP follow target test.

I tried to run the file” follow_target_example.py” but can’t figure out the “root_joint” path problem.(pic below)

the only one clue is that when I’m tried to use the lula test widget, I saw the “root_joint”, in the selected Articulation, but in the ur 20 prim path there is no path called “root joint”

I’m confused — if the prim path doesn’t show a “root_joint”, then where does the error mentioning “root_joint” come from?
please help.

You may want to check out this related post: Adding a New Manipulator example doesn't work - #13 by zakydzul. It covers similar issues with prim path errors and missing joints when adapting manipulator examples in Isaac Sim.