in src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs [775:782]
private void GiveErrorForMissingFramework()
{
var missingFrameworkErrorString = IsLockFileProjectJsonBased(ProjectLockFile) ?
nameof(Strings.MissingFrameworkInProjectJson) :
nameof(Strings.MissingFrameworkInProjectFile);
ThrowExceptionIfNotAllowingFallback(missingFrameworkErrorString, TargetMonikers.First().ItemSpec);
}