The current .NET SDK does not support targeting .NET Core 3.0 | 3.1 | 5.0 | 6.0

asp.net .net .net-core msbuild

You also have to update your visual studio to the minimum supported version:

.NET Core 3.0

minimum visual studio v16.3

.NET Core 3.1

minimum visual studio v16.5

.NET Core 5.0

minimum visual studio v16.6 latest preview

.NET Core 6.0

minimum visual studio v17.0 latest preview

(https://dotnet.microsoft.com/download/dotnet-core/3.0)

(https://dotnet.microsoft.com/download/dotnet-core/3.1)

(https://dotnet.microsoft.com/download/dotnet/5.0)

(https://dotnet.microsoft.com/download/dotnet/6.0)