Your itinerary cannot be saved because a page was modified by Safari Portal and also within your itinerary at the same time. This is a very unusual occurrence and here's what you can do to make sure you don't lose your work:
Step 1. Open Safari Portal (account.safariportal.app) in another browser window.
If you are working in Chrome for example, open Safari Portal in Firefox or Safari browser.
Step 2. From the Itineraries dashboard, click Edit on the same itinerary you have been having the issue with.
Step 3. Copy and paste any new edits you made since your last save from the original browser you had open to this new one.