As renojim stated, you need a source directory/folder in your zip to hold any brightScript files, so make sure to start the zip at that level.
I believe the minimum required in the zip to actually have a working package is:
/manifest
/source/script.brs
where script.brs is whatever you want to name it (keeping the extension), and needs to define one of the acceptable "main" functions used as the initial execution point of the program when run. You may have to define the icon images in the manifest for it to be valid (?), but the images don't have to exist if you are just starting...
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