поправил пути

LR1/AppTesting/AppTesting.csproj

@@ -8,4 +8,11 @@
     <StartupObject></StartupObject>
   </PropertyGroup>
 
+<ItemGroup>
+  <None Include="..\TestData\**\*.*">
+    <Link>TestData\%(RecursiveDir)%(FileName)%(Extension)</Link>
+    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+  </None>
+</ItemGroup>
+
 </Project>

LR1/AppTesting/Program.cs

@@ -4,7 +4,7 @@ public class Program
 {
     private static void Main()
     {
-        double[] inputArray = ReadData(@"..\..\..\..\TestData\input_data.txt");
+        double[] inputArray = ReadData(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "TestData", "input_data.txt"));
         Console.WriteLine($"Input data: [{string.Join("; ", inputArray)}]");
 
         var outputData = ArrayProc(inputArray);