A plugin could be interfering here. Try deactivating all plugins and then check whether saving works. If so, one of the plugins was to blame.
Your theme could also be to blame. Switch to a classic standard theme such as TwentyTwentyOne as a test and see if it works.
A server-side error could also be the cause. You can check whether this is the case in your hosting provider’s error log. Your hosting provider’s support team can help you find it.
Many thanks. I have the site working using a different theme.
— Gary