(please specify the URL of the site on which you are experiencing the problem)
The REST API is one way that WordPress and other applications communicate with the server. For example, the block editor screen relies on the REST API to display and save your posts and pages.
When testing the REST API, an unexpected result was returned:
Please especially note the article that @KangJL shared. Because the WordPress REST API not being reachable is not necessary a problem that will cause problems for your website. Most WordPress websites on our hosting work just fine with this limitation.
Contrary to what the error message from Site Health suggests, while WordPress cannot query the REST API itself, your browser can, which means features like the block editor will work without any issues.
Please don’t waste your time trying to fix an issue that someone says there is but isn’t actually an issue.