When I try to use the autorig, I get a string of errors. I've only opened a new scene, added a biped auto rig, and clicked create new rig. Then the attached error occurs. Any thoughts on fixing or am I doing something wrong?

Hmmm, same here in 15.5.607
in 14.x it was working

Best solution is maybe to report it:
https://www.sidefx.com/Support/report-a-bugrfe/ [sidefx.com]

Yeah, it is also erroring out on my end. H15.5.607

Although I found a bit of a workaround. I'm just not sure if it is bulletproof since I haven't really used Auto Rigs in Houdini.

1. Create a Biped Auto Rig (Shelf tool or Tab Menu)
   
2. RMB on it and open up the Type Properties menu.
   
3. Over along the top, at the Save to Library field, set it to Embedded.
   
4. Hit Accept. (Unlock and Save)
   
5. RMB again on the biped_auto_rig geometry, then select Operator Type Manager. *Close the dialog*
   
6. Now, on the Biped Auto Rig parameters pane, hit the Create/Modify/Match button.
   

>> It should now allow the creation of rigs.

Note, as mentioned in the help card, you may need to Refresh Imports in order to remove the path errors or missing parameters.

* #5 is kinda weird.. if I skip this step, the biped creation would hang my Houdini session. I guess after opening the Operator Type Manager dialog, you may as well check to see if your Object/biped_auto_rig is indeed Embedded.