If ANYONE is getting a similar issue, in the moder era (2025), especially building apps on Mac OS in Rider, or likely, anywhere else, there is a conflict between the SDK and Runtime Versions that seems to be causing the problem. Just create a global.json file at the root of your project (in the same folder as your .sln file) and add the following code. At least, this applies to .NET 9 (12/03/2025).

{
  "sdk": {
    "version": "9.0.300",
    "rollForward": "latestFeature"
  }
}