When you see that image, it means that you're running into a self-signed certificate for WHM, that you'll have to accept in order to proceed.

Sometimes you neeed to import the certificate from a server to another to enble SSL and not display this message. The PFX file enables transfer of certificates and their private keys from one computer to another. Assume, you have a file named domainexample.com.pfx and do you want to install into a WHM / cPanel

1. Export the key file,

openssl pkcs12 -in domainexample.pfx -nocerts -out domain.key

2. Export the decrypted key file,

openssl rsa -in domainexample.key -out domain.decrypted.key

3. Export the Certificate, crt file,

openssl pkcs12 -in domainexample.pfx -clcerts -nokeys -out domain.crt

4. Export the Ca Bundle file,

openssl pkcs12 -in domainexample.pfx -cacerts -nokeys -out cabundle.pem

After all these operations, go to Open WHM and follow the menù

Home >> SSL/TLS >> Install an SSL Certificate on a Domain