Don't worry! We have a solution for you!
This error happens when you are converting a Sales Gallery into an Online Gallery. A Sales Gallery needs to have a message template attached. It's what guides your clients through their Sales Gallery. Because of this, we assign a message template for you when you create a Sales Gallery.
This message template, isn't compatible with an Online Gallery, so when you change the toggle to "Online," it throws an error.
To fix this error, remove the Message Template from the center of your Gallery Settings page. Click Save when you finish, and you won't receive the error anymore!