Somehow wp-json is not working in your website.
Do you use any security plugin? If yes, Please deactivate all of them, Then test. Then activate one by one.
Also deactivate any custom code plugin if you have.

Let me know.

Please deactivate all plugins except this one and change theme to one of default theme (twenty*) and default htaccess then test.
Then activate one by one.

Let me know.

To debug further i will need to access your website. I need admin and FTP (both) access. Please send using https://www.shamimsplugins.com/sensitive-information/

Also please give me a time when you will be available so that i can live chat with you when debug.