URDF models break in Rviz when countinous joints are added, Joint GUI interface blank

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Using ROS Noetic on Ubuntu 20.04.06

I'm learning ROS, and currently following along with this tutorial. The previous example/model (05-visual.urdf) shows up fine in Rviz. (I don't have enough points to add pictures)

Additionally, across all the models the joint-state-publisher window which should pop up a GUI that allows me to control the values of all the non-fixed joints, is not showing the proper interface (it's just a white window vs what the tutorial shows with various sliders).

But the 06-flexible.urdf model doesn't load correctly in Rviz (all the links connected to the head, [including the head] are centered on the map/base-link origin. But if I set all the joints in 06-flexible.urdf from "continuous" to "fixed", it goes back to showing up fine in Rviz. Here's what shows up in the terminal when Rviz is loaded:

Traceback (most recent call last):
  File "/opt/ros/noetic/lib/joint_state_publisher_gui/joint_state_publisher_gui", line 52, in <module>
    jsp_gui = joint_state_publisher_gui.JointStatePublisherGui("Node: " + rospy.get_name(),
  File "/opt/ros/noetic/lib/python3/dist-packages/joint_state_publisher_gui/__init__.py", line 103, in __init__
    slider.setValue(RANGE/2)
TypeError: setValue(self, a0: int): argument 1 has unexpected type 'float'
[joint_state_publisher-2] process has died [pid 11135, exit code 1, cmd /opt/ros/noetic/lib/joint_state_publisher_gui/joint_state_publisher_gui __name:=joint_state_publisher __log:="insert/path/to/rest/of/log/here"

But sometimes I get this error instead:

libGL error: MESA-LOADER: failed to open iris: /usr/lib/dri/iris_dri.so: cannot open shared object file: No such file or directory (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri, suffix _dri)
libGL error: failed to load driver: iris
libGL error: MESA-LOADER: failed to open iris: /usr/lib/dri/iris_dri.so: cannot open shared object file: No such file or directory (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri, suffix _dri)
libGL error: failed to load driver: iris
libGL error: MESA-LOADER: failed to open swrast: /usr/lib/dri/swrast_dri.so: cannot open shared object file: No such file or directory (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri, suffix _dri)
libGL error: failed to load driver: swrast

In both cases I also get

 TIFFReadDirectory: Warning, Sum of Photometric type-related color channels and ExtraSamples doesn't match SamplesPerPixel. Defining non-color channels as ExtraSamples..
TIFFFieldWithTag: Internal error, unknown tag 0x829a.
TIFFFieldWithTag: Internal error, unknown tag 0x829d.

Followed by a few dozen more of the "TIFFFieldWithTag: Internal error, unknown tag 0x000a" lines. From some research, this TIFF error I can ignore.

But the other errors that show up have got me stumped. The joint_state_publisher_gui package is up to date, so i don't think it's directly at fault. Poking around the log files (to keep things short I won't list it all here) the only thing my beginner skills catch is

/use_sim_time is not set, will not subscribe to simulated time [/clock] topic
in joint_state_publisher-2.log

Some poking around suggested using and online searches suggested

rosparam set use_sim_time true

but ... (more)