Unable to verify the first certificate in api

I am trying to add an api using API connector plugin. But, when I initialize the call. I get this error “unable to verify the first certificate”. Please, help me. I have read some related problems and some are saying that I need DigiCert SHA2 Secure Server CA certificate.

But, I already have that certificate for my domain. Still, I am getting this error. Please, help me.

I have already tried this “rejectUnauthorized: false” header. But, still no luck.