A misleading error message "Unable to add layer [Web Map Name]. The layer, [Web Map Name], cannot be added to the map" is returned when saving a web map in Map Viewer. The error message is prompted when uploading custom images for symbolizing unique values.
This issue is not reproducible after adding more than ten additional images and following the reproduction steps. It is recommended to publish these symbols as a web style so this gallery can be on hand and not have to upload each one individually and can be shared across the organization.