Xamarin.Helpers | Several Xamarin-based classes

NOTE: None of the following answers "how to target .NET Framework 4.6.1 for a Xamarin.Forms project". If you really, truly, need to know how to do that, and know what you are doing, and why, then search elsewhere - doing so is an advanced topic, and is no longer recommended. For cross-platform projects, stick with a .NET Standard version.

In this case, despite the question title, doing so is not the solution. Keep reading for more details.

I'm trying to use a nuget package targeting .NET Framework 4.6.1 and it doesn't load because of that.

Actually, it does load. It gives you a warning. You can probably ignore that Framework vs Standard warning - I've been ignoring a Framework 4.6.1 vs Standard 2.0 warning for over a year, without problem.

error .. saying it is not compatible with monoandroid71

This is the real problem that is stopping you.
Is there a packages.config file? If so, open it in a text editor, and search for monoandroid71. If present, then yes, this is the problem -
The packages.config mechanism contains an outdated "hint", OR you ARE targeting an older Android version, that is not compatible with the nuget you wish to use - as the message says.

Possible fix:

• Target Android 8.0 or higher. In your Android project (e.g. if you named your Xamarin Forms project "App1", look in solution for the project labeled "App1.Android") / Properties / Application / Compile using Android version (Target Framework) / select Oreo or Pie.
• Update all nugets to latest.
• If packages.config still mentions monoandroid71, then you have outdated "hints".

Different ways to fix outdated hints in project.config. Do ONE of the following:

• Rt-click on project.config in Solution pane, select "Migrate to PackageReferences". This newer mechanism doesn't rely on hints.
• OR Delete project.config. Add the nugets again.
• OR Manually edit project.config. Change all targetframework=monoandroidXX to be the same - whatever the highest number you find in the file. Probably monoandroid81 or monoandroid90.
• OR Start all over. Latest VS 2017 version OR VS 2019. Either works fine for this purpose. New project / C# / Mobile App Xamarin Forms. Copy your source files (.cs, .xaml) to the new projects - DON'T use your old .csproj or package.config. The goal is to use all the latest build options, including PackageReferences.