For anyone running into "Failed" errors when trying to publish your custom track, please follow these steps:
1. Opt into the 'testing' beta branch from the Steam library properties of Rift of the NecroDancer Level Editor
2. Click the "Save as" button in the editor's top toolbar, select "Save as new track" and enter a slightly different name from before (e.g. by appending "2" to the name)
3. Select a cover image that is smaller than 1 MB and in the `png` image format
4. Click "Publish track", accept the Steam Workshop Legal Agreement, and click "Upload"

If you cannot publish still with this, please leave your log files in this thread! You can find those log files in the install location of the editor. Thanks!