C# - why costura and fody doesn't work to me?

I have this warn, too. But it works fine for me. So it's not the warn that leads to your issue.

In my opinion, maybe it's because the costura version does't fit your project. You can try an older version.

As for me, my project is using .NET Framework 4.6, and the latest costura.fody 5.3 version isn't compatible. But the 5.2 work fine.

And how to remove the warn: (edit the csproj file)

1. Right-click on your project in solution explorer and select Unload Project Right-click on the project (tagged as unavailable in solution explorer) and click "Edit yourproj.csproj". This will open up your CSPROJ file for editing.
2. find below and comment "IncludeAssets" line as below(by adding <!-- -->)

    <ItemGroup>
    <PackageReference Include="Costura.Fody">
    <Version>5.2.0</Version>
    <!--<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>-->

3. After making the changes, save, and close the file. Right-click again on the node and choose Reload Project when done.

Looks like previous answer, I got rid of the warning this way:

1. Double click on project node
2. In CSPROJ file add "compile;":

    <IncludeAssets>runtime; compile; build;.....

3. Reload project

Actually, the error message tells you what to do: "... it's recommended to completely remove IncludeAssets".

More specifically, in your project file ( .csproj or .vbprog ) find where it says as an example:

  <PackageReference Include="Fody">
  <Version>6.3.0</Version>
  <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
  <PrivateAssets>all</PrivateAssets>
  </PackageReference>

and change it to:

    <PackageReference Include="Fody">
    <Version>6.3.0</Version>
    <PrivateAssets>all</PrivateAssets>
    </PackageReference>

(i.e. remove the <IncludeAssets .....> line entirely).

Doing this, as applicable, for both costura and fody.