I cannot install the System.Objectmodel reference in the Android Project

Question

I picked up a reference error in my project The referenced component 'System.ObjectModel' could not be found.. Well, I searched and installed the reference to that path:C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.ObjectModel.dll. Well, when I finished installing, without doing anything, this error appeared:

Project App2.Android cannot build using Android 7.1 (API Level 25 - Nougat) because Android SDK Platform for API Level 25 is not installed. Either install it in the Android SDK Manager (Tools->Android->Android SDK Manager...) , or change your Xamarin.Android project to target an API version that is installed.
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And just below, saying that the reference was not found(recem installed).

The referenced 'System.Objectmodel' could not be found. App2.Android

I’m researching and I haven’t found anything yet. Has anyone gone through this and could you share? I will install the android 7.1, but what I find more strange is not finding the reference.

Edit 1

Well, I uninstalled the reference(dll) and reinstalled it and nothing, it continues with that yellow exclamation. I removed again and got a link on the internet that recommends doing the installation, via Nuget. I did and now the reference does not appear in the reference list and when I rotate, continues with deploy error, but the Warning that the reference was not found, disappeared, but is without the said whose reference. The version by Nuget is the 4.3.0 and I don’t know the impact of it. This is the message that gives output:

1>Starting deploy Visualstudio_android-23_x86_phone ...
1>Starting Visualstudio_android-23_x86_phone ...
1>C: PROGRA~2 Android ANDROI~1 tools Emulator.EXE -Partition-size 512 -no-boot-anim -Avd Visualstudio_android-23_x86_phone -prop monodroid.avdname=Visualstudio_android-23_x86_phone

1>Emulator Visualstudio_android-23_x86_phone cannot be Started.

========== Deploy: 0 succeeded, 1 failed, 0 skipped ==========