My ssl certificate generates with lets encrypt but does not install

Question

I'm using [this][1] library to generate SSL certificates. My storage generates 4 files: certificate.pem, private_key.pem, chain.pem, and fullchain.pem.

I want to install this certificate in acquia cloud using their Rest API post endpoint to install ssl certificate. The payload looks like the following:

{
  "legacy": 0,
  "certificate": "pasted the content inside our certificate.pem",
  "private_key": "pasted the content inside private_key.pem",
  "ca_certificates": "pasted the content inside the fullchain.pem",
  "label": "My New Cert"
}

When I send a request, I received an error to contact they api owner support, and searching around through the server log I came across this:

 Error response: 500 (Internal Server Error). Error message: Site certificate CA chain certificates are out of order..

What exactly does this error mean by saying out of order?