Rope should be ɹəʊp but the built-in Oxford Dictionary of English displays it as rəʊp
How can I fix this?
If you don't like something in one of the Oxford dictionaries, you have to contact the publisher and ask for a change. Apple has nothing to do with the content of these dictionaries, they just buy them from Oxford.
The very same ipa is used in the large number of Oxford bilingual dictionaries provided in every install of MacOS and iOS. Plus you can see it online at a site like this one. When you contact Oxford, be sure to provide as much evidence as possible regarding the error you think they have made.