The strange part was that I was not able to see any certificate when trying to assign the certificate !!
After a little bit of investigation I figured that my CA certificate is missed from Lync’s local certificate store
I just simply imported my root CA certificate to Lync’s server local certificate store
Just at this stage my Lync’s server certificate was there and able to be assigned.
I just assigned it and proceeded with my Lync server installation
Lync server certificate is not viewable for assignment !!
I was just installing Lync server and when it comes to the certificate part although I was able to issue the required certificate using Lync server installation wizard , see below :