Already found same problem here, but that not solve my problem.
Exception details:
Severity Code Description Project File Line Suppression State Error The "Xamarin.Forms.Build.Tasks.GetTasksAbi" task could not be loaded from the assembly C:\Users\sonub.nuget\packages\xamarin.forms\2.5.1.527436\build\netstandard1.0\Xamarin.Forms.Build.Tasks.dll. Confirm that the declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask.
I have installed xamarin forms 3.1.0.912540 from 2.5.0.121934 for all the platforms. After that, I got this error. I research about this error and someone said that install .NET Framework 4.7.1 would help solve this issue.
But after installing .Net Framework getting the same error. Installer screenshot after installing .Net Framework.