If you receive this error, it means your session has expired and you are logged out from your SwipePages account. Please do not close the current browser tab in which you have Swipepages editor open.
Open a new browser tab and login to your SwipePages account . Once logged in, come back to the tab where you have the builder open, you should now be able to save your work without issues.
Please have a look at this video to get a better perspective.