Updating old xamarin.forms app

Hi,

I have this old app Xamarin.forms (Android/iOS). It was developed with xamarin.forms 2.x. Now I must update it.

I convert the old PCL to a net.standard 2.0 library.
However, when it start it throws

Can't find custom attr constructor image: .dll mtoken: 0x0a000074 due to: Could not resolve type with token 0100004e from typeref (expected class 'Xamarin.Forms.Xaml.XamlResourceIdAttribute' in assembly 'Xamarin.Forms.Core, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null') assembly:Xamarin.Forms.Core, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null type:Xamarin.Forms.Xaml.XamlResourceIdAttribute member:(null)
Loaded assembly: netstandard.dll [External]
Unhandled Exception:

System.TypeLoadException:

I clean obj and bin folder and I disable XAML compile by putting these line of code

#if !DEBUG
     [XamlCompilation(XamlCompilationOptions.Compile)]
#endif

Above the class with that attribute.

Can you help me?