I was able to resolve this by doing these 2 main steps (and maybe a 3rd step of setting up an external storage location):
(1) Create the storage credentials from the Data Catalog
(2) Use Postman to call the REST API to update the metastore with the ID of the storage creds, which also required using the REST API to enumerate and confirm the storage credential unique ID
My only guess is that the AWS Quick Start that created this workspace had some sort of bug in the setup script that prevented it from successfully configuring the metastore. It took several hours of spelunking to get the confidence that this was the right approach.
Note that I did take another step that I did not list above, which was to create a Storage Location within the Data Catalog (see docs in the same place where storage credentials are created) -- but I'm not sure if that was really required and I'm too afraid to remove it now. I did compare the workspace setup between this and another workspace and saw that that credentials were not displayed when viewed from the top-level Databrocks Account UI, which is exactly what this error message is saying.