As J.Dhaik has already mentioned MvvmCross version 5.7.0 has not been updated to support .NET Standard yet. The next major release version 6.0.0 will add support for .NET Standard 2.0.
Using MvvmCross versions prior to 6.0.0 inside of a .NET Standard class library is however possible.
So why the warning?
You can check out the explanation I gave on this Stack Overflow question for why you would see the warning. Extract below
With .NET Standard 2.0 and the updated tooling in .NET Core SDK 2+ the
.NET team wanted to make it easier to update or make use of .NET
Standard libraries. The issue is that not all NuGet packages have been
updated to support a version of .NET Standard. So they introduced a
fallback targeting .NET Framework 4.6.1 which is nearly 100% compliant
with .NET Standard (There are some API that are in the .NET Standard
2.0 spec that are not in .NET Framework 4.6.1 but they can be brought in via NuGet packages if required). So the warning you see is to
inform you that the packages do not conform to a .NET Standard version
you are targeting and as such may contain API's that are not
executable in your runtimes making use of your .NET Standard 2.0
library.
How to suppress the warnings
NuGet provides two options, per package or project level.
Per package
You can edit your csproj and add NoWarn="NU1701" tag to your package reference or select properties of your NuGet package refernce (Solution Explorer > Dependencies > NuGet > {The package name} right click Properties) and add NU1701 to the NoWarn property.
The result would be similiar to the following in your csproj
<ItemGroup>
<PackageReference Include="MvvmCross" Version="5.7.0" NoWarn="NU1701" />
<PackageReference Include="MvvmCross.Core" Version="5.7.0" NoWarn="NU1701" />
<PackageReference Include="MvvmCross.Binding" Version="5.7.0" NoWarn="NU1701" />
<PackageReference Include="MvvmCross.Platform" Version="5.7.0" NoWarn="NU1701" />
<PackageReference Include="MvvmCross.CodeAnalysis" Version="5.7.0" NoWarn="NU1701" />
</ItemGroup>
Note, with using the per package approach dependency package warnings are not suppressed by suppressing the parent package. So you would need to bring in the package as a dependency in order to suppress the warnings.
Project level
NuGet also gives you the option to suppress all NU1701 warnings at a project level. You can do this via manually editing the csproj as follows
<PropertyGroup>
<NoWarn>NU1701</NoWarn>
</PropertyGroup>
Or via the GUI you could modify Suppress warnings to include NU1701
