Excel Custom Functions using Add-Ins: "This add-in is no longer available" error

Question

I followed all of the steps here to try and load in a basic excel custom function add in. Everything in the setup stages seems to work, however, when I run 'npm run start:desktop' and Excel opens, there is an error on the side that says:

ADD-IN ERROR This add-in is no longer available: add-ins inserted during development are only available during debugging from Visual Studio. Please open your project in Visual Studio and re-run your application, or deploy your application into a valid catalog and re-insert

I get this error whether I run the project from the command line, VS Code, or Visual Studio. I am on Windows 10 using Excel 2016.

Here is a photo of the Excel page with the error:

Answer 1

Clearing the Office cache sometimes fixes this: Clear the cache

UPDATE 1/23/2021:

There is now a troubleshooting section for this error: Getting error "This add-in is no longer available".