尽管已安装软件包，但无法找到 NuGet 软件包 NU1101

Question

I created my first C# application recently, and since buying a new computer I cannot get the application to start. I am receiving 7 of the following errors Error NU1101 Unable to find package Microsoft.VisualStudio.Web.CodeGeneration.Design. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages clesson C:\Users\porkh\Desktop\WERK\Personal\c-lesson\clesson\clesson\clesson.csproj Error NU1101 Unable to find package Microsoft.VisualStudio.Web.CodeGeneration.Design. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages clesson C:\Users\porkh\Desktop\WERK\Personal\c-lesson\clesson\clesson\clesson.csproj Each with different packages missing.
Since this was a missing dependencies message I reinstalled visual studio 2019 with C# .net support again, yet I am running into the same error.
On StackOverFlow I read that I need to check my NuGet packages,

Here it shows every package that is "missing" in the error messages as installed.
I have also tried running dotnet run and dotnet restore but I get the same error messages. I am extremely new to C# so sorry if I have missed the solution in other's answers. Any help you can give me I'd appreciate it.
A link to all of my project files here

Answer 1

In the image it says

Not available in this source

and the package source is set to Offline Packages. My best guess is, switching to an official NuGet Feed in the drop down to the upper right, will solve your problem. You also want to check, if the Nuget source is available (it should). If not, click the Cog icon, and add it there with the name "Nuget" and the Url https://api.nuget.org/v3/index.json