Made Model an ViewModel projects class libraries.

CSIP-42.ThemePark.Model/CSIP-42.ThemePark.Model.csproj

 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <OutputType>Exe</OutputType>
+    <OutputType>Library</OutputType>
     <TargetFramework>net5.0</TargetFramework>
+    <ApplicationIcon />
+    <StartupObject />
   </PropertyGroup>
 
   <ItemGroup>

CSIP-42.ThemePark.Model/Program.cs

-using System;
-
-namespace CSIP42.ThemePark.Model
-{
-    class Program
-    {
-        static void Main()
-        {
-            Console.WriteLine("Model says hello");
-        }
-    }
-}

CSIP-42.ThemePark.ViewModel/CSIP-42.ThemePark.ViewModel.csproj

 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <OutputType>WinExe</OutputType>
+    <OutputType>Library</OutputType>
     <TargetFramework>net5.0-windows</TargetFramework>
     <RootNamespace>CSIP_42.ThemePark.ViewModel</RootNamespace>
 	  <UseWPF>true</UseWPF>
+	  <ApplicationIcon />
+	  <StartupObject />
   </PropertyGroup>
 
   <ItemGroup>

CSIP-42.ThemePark.ViewModel/Program.cs

-using System;
-
-namespace CSIP_42.ThemePark.ViewModel
-{
-    class Program
-    {
-        static void Main(string[] args)
-        {
-            Console.WriteLine("Hello World!");
-        }
-    }
-}