problem in redirection after clicking on core function

Hello,
My website link: http://www.sonicbiochem.co.in/
I am experiencing a problem when i click to “www.sonicbiochem.co.in/api/get_page_index/” the core functions it does not
returns the result in json but it redirect to the page http://www.sonicbiochem.co.in/certificate-of-registration-22000/
i want to ask why it is redirecting to page instead of giving the json code.

The same problem with all of the function of core controller.

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