Hello MarkNelson,
Thank you for using the Apple Support Communities. If I understand your post correctly, you're running into some issues working with a podcast file made in GarageBand.
To clarify a few points, which format did you export to? For example, .m4a, .m4v, or something else? For reference, this has some information about enhanced podcast formatting: GarageBand for Mac: Enhanced Podcasts (.m4a, .m4v). Depending on the format you used when exporting, you might try re-exporting with one of those formats.
Which macOS version are you using?
When you say you're uploading or importing the file to Apple Music, are you referring to adding it to the Music app in macOS Catalina? Or are you trying to upload this as a podcast that people can download?
I hope this helps get you started. Feel free to reply back with more detail, and the community may be able to further help out.
Take care.