Probably because they aren't authorized on GitHub.
Here's a link:
https://ci.appveyor.com/project/MediaPortal/mediaportal-1/builds/54946065/job/jwqg1caoknpv7x0q/artifacts
in about 20 to 30 minutes, an artifact named Release\MediaPortalSetup_x64_1.39.Final... will appear, and you'll be able to...
That's strange; you can download it from this link https://github.com/MediaPortal/MediaPortal-1/raw/refs/heads/MP1.39.1-Lame_enc_4.0/mediaportal/MediaPortal.Base/Lame/x64/lame_enc.dll
I don't know if there have been any changes to this plugin over the past year. If there are any issues, you should probably describe them here and provide log files; then there's a chance they'll be fixed.