Hey!
I encountered this issue when running on a Mac with an M1 chip. I found this to be of great help Cannot build .tmx files. TiledMapTilesetProcessor showing a MissingMethodException · Issue #783 · craftworkgames/MonoGame.Extended · GitHub.
After I set up the new Nuget source and downloaded the beta versions of the nuget packages the application started as expected. I also had to ensure I was running the x64 variant of .NET SDK and not the armx64 as that is not yet fully supported by the chip. (MGCB Not Launching on macOS M1)
Hope that helps!