Problem: The current .Net SDK does not support targeting .NET Core 3.0. Either target .NET Core 2.2 or lower. (Visual Studio 2019 unable to locate .Net Core SDK)
Note: Your IDE or Visual Studio 2019 is looking into the 64-bit version of Windows for DotNet Core 3.0 or the other way around.
Solution: Go to Microsoft website and download the version of .Net Core 3.0 or 2.2 according to the one you want. Make sure the version of the Visual Studio 2019 is supported for the .Net Core version you are downloading. https://dotnet.microsoft.com/download/dotnet-core/