Problem
The following error message occurs
"Server certificate specified is unusable"
Cause
It is due to an incorrect installation of a certificate through the Microsoft management console (MMC), instead of the IIS installation wizard. This error occurring when assigning a certificate to a Citrix gateway.
Resolution
If the pending request exists within the IIS installation wizard, delete the certificate installed through Microsoft management console (MMC) and install the certificate through the IIS installation wizard. Once installed, assign the new certificate to the Citrix gateway.
OR
If not, delete the certificate installed through MMC
Then, you will have to create a new CSR and reissue your certificate in your MySSL account.