Dear J K 0328,
Good day! Thank you for posting to Microsoft Community. We are happy to help you
According to your description, we know that you are unable to save the modified SharePoint page and receive an error message at the same time.
According to my investigation, I find that the situation encountered in this thread is very detailed with your description. Users find this phenomenon is related to the change of the page thumbnail. I would like to know whether you have changed the page thumbnail, if so, please refer to this thread first:
Modern bug: Users can't save pages - Microsoft Community

If you are still experiencing this phenomenon, I would also like to know the following information:
- Whether this phenomenon occurs to all users?
- Whether this phenomenon occurs in all Sites?
Sincerely,
Tin | Microsoft Community Moderator