Cannot enable SSL in windows 2012 server

I find it easy to import externally generated SSL certificates using openssl to create a private key, and a certificate request, then fulfill the request from either a Microsoft Certificate Server or from a commercial provider.

I convert the key and certificate to pfx format and copy it to the server. The IIS Server Certificate menu shown in your link easily imports the certificate.

You can also use mmc with the certificates snap-in to import the pfx certificate into to roles you want to use it for. Here are the commands.

Launch OpenSSL application
    C:\OpenSSL-Win32\bin>openssl

set OPENSSL_CONF=c:\OpenSSL-Win32\bin\openssl.cfg
HOST = your hostname
    
PROCESS

STEP 1: Generating RSA private key for CA (2048 bit)

    openssl genrsa -des3 -out HOST.key
    
STEP 2: Generating X.509 certificate signing request for CA

    openssl req -new -key HOST.key -out HOST.csr

STEP 3: Generate HOST cetificate from Microsoft Certificate Service on CA or from commercial provider. Cut and paste from HOST.csr
    
    Download new certificates  HOST.cer
    
STEP 4: Create PEM Certificate (PKCS12, PFX)

    openssl pkcs12 -export -out HOST.pfx -inkey HOST.key -in HOST.cer