Probably you are using code example from extension page…
Message "Before add photos to product gallery, you need to save product" - means that there is no record in db for gallery associated with model(it will be shown in case when it is new model either this model never was saved with galleryBehavior).
To fix this you should save model with gallery bahavior at least one time - this will add row for that gallery into db, and you will be able to manage it.