@Yin_Ar You may want to move over to forums.realm.io for this question - there is support for building a Unity app, with our Realm .NET SDK, and using Atlas Device Sync to sync to Atlas. There are some docs here -
https://www.mongodb.com/docs/realm/sdk/dotnet/unity/
But the error you are seeing is that you are trying to connect with a partition-based sync API from the client but your server-side Atlas App Services is configured with Flexible Sync - you’ll want to change it on one side or the other but I’d recommend Flexible Sync as that is the future of Device Sync at MongoDB.