NU5039 The readme file ‘README.md’ does not exist in the package

This appears to be quite a common issue and it happened to me unexpectedly when trying to pack a legacy library. The library concerned was written in net core 3.1 and has recently been upgraded to .net 8.0

The first thing I tried was adding

   <IsPackable>true</IsPackable>

to the csproj file but this made no difference. I also checked the CopyToOutputDirectory setting.

Finally I noticed that I had

    <PackageReadmeFile>README.md</PackageReadmeFile>

and ALSO had

  <ItemGroup>
    <None Update="README.md">
      <Pack>True</Pack>
      <PackagePath>\</PackagePath>
    </None>
  </ItemGroup>

Removing the PackageReadmeFile entry resolved the issue for me