go to Settings > Permalinks and Click Save Changes, this will rebuild the sites permalinks and kick start the REST API.
Then retry updating the page.
If the issue persists, whilst editing that page, Right Click > Inspect it to open the Browser Developers Tools. Go to its Console tab, and look for any errors whilst it is trying to uddate
Remove the Gutenberg Plugins, and don’t re-install it, that plugin is not required and is only for developer testing, its effectively a permanent BETA version of the block editor for devs to check out and contribute to future features and bug testing…