Collector is not allowing me to download a basemap and brings me to a screen that says " Secure Layer This layer is protected: Unkown. https://tiledbasemaps.arcgis.com/arcgis/rest/services/World_Imagery/MapServer '" Then asks for Username and Password
see screen shot attached.
You will need to add an Esri Basemap to your Portal. This requires using ArcGIS Online credentials to store with the layer. Once that is done, switch the basemap you have with the one you just added.