Sometimes using a 301 redirect plugin can work around this issue.
It is also worth noting that if you have a deleted post or page in the Bin area ( At the top section) This will need to be cleared out of the bin, if it holds a page / URL that you are looking to redirect to.
Apart from the above, the post itself should be editable via the Permalink / URL. Then you can add a redirect rule via a 301 redirect plugin from the old URL to the new cleaner one.
Hope this helps
Redirection – WordPress plugin | WordPress.org English (UK)[
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