The type or namespace name 'Android' does not exist in the namespace 'Xamarin.Forms.Platform'

Hi guys.
I Updated my nuget packages few days ago to new versions.
but my Android project has 2 errors in MainActivity.cs:
1:
The type or namespace name 'Android' does not exist in the namespace 'Xamarin.Forms.Platform' (are you missing an assembly reference?) Sama.SamaApp.Android MainActivity.cs

2:
'MainActivity.OnCreate(Bundle)': no suitable method found to override MainActivity.cs

i search it in google and find solutions.

But i tested all solutions in two up links but not worked for me.
i tested clear nuget and dotnet caches . removing bin & obj folders . restore nuget packages . remove and reinstall Xamarin.Forms .finally clean rebuilding the solution.
But I could not fix it.
thanks...