Headers are not accepting

I am trying to download certificate from ionic from my wordpress Leardash plugin.
In chrom’s post man using url https://key2know-learning.dk/certificates/certificate/?course_id=324&cert-nonce=fcb9753683
and adding headers Authorization. And I am able to get certificate.

But when I try to access same from Ionic app I am getting error Access to XMLHttpRequest at ‘https://key2know-learning.dk/certificates/certificate/?course_id=324&cert-nonce=542e9d5993’ from origin ‘http://localhost:8100’ has been blocked by CORS policy: Request header field authorization is not allowed by Access-Control-Allow-Headers in preflight response.

This is not CORS Access-Control-Allow-Origin error. Because I am able to get my posts and courses and all. But here in error clearsly says it is “Access-Control-Allow-Headers” error.

Please help.