Hi @Marek Forys ,
Welcome to Microsoft Q&A!
Please open your project file and check if the <PackageReference /> items is declared without Version attributes.
<PackageReference Include="Newtonsoft.Json" />
And go to Directory.Packages.props file and check if you have set the ManagePackageVersionsCentrally property to true.
<Project>
<PropertyGroup>
<ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
</PropertyGroup>
<ItemGroup>
<PackageVersion Include="Newtonsoft.Json" Version="13.0.3" />
</ItemGroup>
</Project>
You can restore packages and rebuild your solution.
Sincerely,
Anna
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