Per the Troubleshooting & FAQ section on the Wiki, this is because of the source data from JustWatch. If you go to their website and search for the program, you can see that in the Stream (aka subscription) options, Hulu is listed, but Amazon Prime is not:
Notably, even though in SLM and here you see the "Prime Video" icon, it is not available on the provider "Amazon Prime Video". This program is available on Amazon for purchase, so that would be the provider "Amazon Video" if you did that.
SLM is completely dependent upon JustWatch having correct data. If you find an issue like this, just follow the directions in the link above to submit the issue to them. Potentially, while waiting for a fix, you could manually override the Stream Links, but I'm sure you'd rather not for something with so many episodes.
However, I did just check, and New Girl is on Hulu/Disney+. I'm wondering if there was a slight gap for some licensing reasoning or the other. If you are using an Android TV device, I have found Hulu has killed the ability to accept deep links, so if you click to launch it, nothing happens (a different error also discussed on the Troubleshooting & FAQ wiki page). You pretty much have to do the Hulu to Disney+ conversion now to get it to work there.
