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The program was created in vs 2015 and has worked fine ever since.
That could mean it was created with a different .NET version, of the MSBuild or the solution file is maybe not supported in this version of Visual Studio. Note that the Visual Studio Installer does not install everything, and you must select the components manually to set them up.
Here is what you can do: create a new .NET (Core) project and run it. If it works, the problem is with the .NET version; check which version is used with the project.
If the new project does not work, you must remove the installation and reinstall the entire .NET stack for Visual Studio.
.NET Framework installation guide - .NET Framework | Microsoft Docs[
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