Program does not contain a static 'Main' method suitable for an entry point

Question

I have a Class Library project, say U in which I have written unit tests for project A. I did not add Main() in this test project. When I build U on Visual Studio 2019, it builds successfully and even runs the test cases as expected. But when I try to build this project using msbuildVersion=14.0, it gives CSC : error CS5001: Program does not contain a static 'Main' method suitable for an entry point

Below is U.csproj file:

<Project Sdk="Microsoft.NET.Sdk" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <PropertyGroup>
    <TargetFramework>net461</TargetFramework>

    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <PlatformTarget>x86</PlatformTarget>
  </PropertyGroup>
  
<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
  </PropertyGroup>
  
  <ItemGroup>
    <PackageReference Include="NUnit">
        <Version>3.12.0</Version>
    </PackageReference>
    <PackageReference Include="NUnit3TestAdapter">
        <Version>3.16.1</Version>
    </PackageReference>
    <PackageReference Include="Microsoft.NET.Test.Sdk">
        <Version>16.5.0</Version>
    </PackageReference>
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\..\A\A.csproj" />
  </ItemGroup>

  <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
</Project>

I am unable to understand what should I correct in U.csproj file so that it doesn't try to find Main function and build successfully. Can anyone suggest the required changes?