This error message usually indicates that there is a problem with the navigation settings or structure of the site, and SharePoint is unable to save the changes you're trying to make. Here are a few steps you can try to resolve the issue:
1. Clear your browser cache and try again: Sometimes, issues with the SharePoint UI can be caused by cached data in your browser. Try clearing your browser cache and then refreshing the page to see if the issue is resolved.
2. Check the navigation settings: Make sure that the navigation settings for the site are configured correctly. You can do this by going to the site settings and clicking on "Navigation" under the "Look and Feel" section. Check that the navigation settings are configured correctly and that there are no issues with the structure or links in the navigation.
3. Check for custom code or solutions: If you have any custom code or solutions deployed on the site, it's possible that these are causing the issue. Try disabling any custom code or solutions and then try updating the navigation again.
4. Use PowerShell to update the navigation: If the above steps don't work, you can try using PowerShell to update the navigation settings for the site. You can use the Set-PnPNavigation cmdlet in the SharePoint Online Management Shell to update the navigation settings. Make sure to test any changes in a development or test environment before making changes in production.
If none of these steps work, it's possible that there may be a more serious issue with the site or the SharePoint environment. In this case, you may want to consider reaching out to Microsoft support or a SharePoint developer for further assistance.