Can't get certificate chain

Hi here, trying to use Cloudflare with an AWS API Gateway. AWS requires me to create import a cert using ACM(AWS Cert Manager). I created a an origin ca certificate in Cloudflare.

I followed the steps here:

and here:

But I still get an error that the certificate chain cannot be validated. How does one create a certificate chain? Any resources on explaining this would also be really helpful!

Here is the certificates you should use as the chain, step 4:

Alternatively you can use ACM to generate a free amazon-issued certificate for your domain and it will work with SSL (Strict) mode just the same as the origin certificate.

Thanks, that totally helps!