header("Content-Security-Policy: frame-ancestors 'self'") gives an error in PHP

I have tried adding header("Content-Security-Policy: frame-ancestors 'self'"); in my php file, but when opening the page I get

The Content Security Policy directive 'frame-ancestors' is ignored when delivered via a element.

I know it is not supposed to be used in meta tags, but is there another way to use the frame-ancestors or I am not sure what I am missing

source https://stackoverflow.com/questions/71562420/headercontent-security-policy-frame-ancestors-self-gives-an-error-in-php