Hello,
For DonwloadKeyRequest, that would be your API key.
As for the error. It took some time but I managed to find out that System.Json has been depreciated from .net standard and possible .net framework.
You can find the dll here, to force your project to support System.Json
If you need help with that, let me know and I can tell you what to do.
So I'm sorry to say but it looks like it is not possible to fix at this time. All though deprecated, the issue was not with System.Jason.dll.
Digging into it I believe that Itch.io's API endpoint has changed, and the key generated only has access to profile info, not game info. I will follow up with Itch and investigate further.
That's what this plugin uses. The token generated during authentication is used to make the API call https://itch.io/api/1/KEY/me. Once that returns a UserID, another call is made to verify the purchase https://itch.io/api/1/KEY/game/GAME_ID/download_keys.
It seems the last API call mentioned has been updated.
I have made a post on the support forum, this is the link if you would like to follow also. Once I have an update, I can update the plugin.
https://itch.io/t/1114371/itchio-api-endpoint-change