Hi Michael,
Thank you for your reply, and I suspected something along those lines might be part of the problem. The trouble is that I can't seem to add a feature layer following these instructions:
Publish hosted feature layers—ArcGIS Online Help | ArcGIS
In Step Two: Create ---> Feature Layer, that's not an option. I can only choose App, Map, or Scene.
I also found this blog post after googling around, but I cannot find My Hosted Data as one of the options on the developer's page: Create an Empty Feature Service for use in ArcGIS Online | Support Services Blog
And then there's this advice, but it only applies to accounts with an organizational affiliation and thus I don't have any of those options:
How to create a hosted feature service | ArcGIS Blog
This is frustrating because I was literally not having this problem two days ago. That's what makes me think that somehow my account isn't fully authorized now, but I have no idea how to check that either.
I am a faculty member at an institution that subscribes to ArcGIS but I'm not a member of the academic unit that has the subscription (I'm in Liberal Arts, not Computer Science.) I did not create an account using my university credentials, but I did list my school as the organization. It seems unlikely that this is the problem but I guess it's a possibility.