Registering custom mobile map package with Portal for ArcGIS to use in ArcGIS Navigator is unsuccessful and returns the following error message, "ERROR: Invalid or unsupported package name map_package. Exiting."
The managemmpks tool is only for registering mobile map packages provided by Esri.
To add custom mobile map packages in a portal, refer to the following link,
https://pro.arcgis.com/en/pro-app/latest/help/sharing/overview/mobile-map-package.htm.