Hello swayam, thanks for your offer to help.
I do not have a hosting provider to work with. I have complete root access to the linux machine this is on.
In a browser, I go to my site, log in as admin, and look at the Site Health Status screen. One of the messages is:
The REST API encountered an unexpected result
…
REST API Endpoint: https://www.reach0.net/wp-json/wp/v2/types/post?context=edit
REST API Response: (403) Forbidden
How can I determine what the problem is and fix it?