Hi, sorry about the issue. I suspect there may be an unsupported HTML tag in your Tour somewhere. See this FAQ: https://storymaps.arcgis.com/en/faq/#question32
You may need to delete the most recent point(s) you've added or quit without saving. For example you may have entered some HTML into the caption of one of your points that isn't supported, especially if you are using the option to upload images directly into the Map Tour Builder (because that option, as it writes to a feature service behind the scenes, supports a more restricted set of tags as you can read in that FAQ). The HTML you entered may display OK in the Builder and not look like it is an issue (such as if you embed audio in a tour created by uploading images) but it will give you that message when you try and save. So it many not be obvious what HTML the save is objecting to, because the Builder displays the HTML correctly.
Rupert
