I don't understand your sentence "it shows all enrolled user in the course and their activities.
then I clicked on user name to get the certificate for that user, it
generate the certificate but Admin name was printed on instead that
user(who avail and completed the course, - I Clicked on)."
As you don't say what version of Moodle and what version of the certificate module activity you use, here is how to do to be able to download certificates for a student in recent Moodle and certificate versions.
Fist thing is that you must set "Save certificates" to "Yes" in the "Issues options" of the certificate settings page whne you create the certificate activity. It will not work if this setting is set to "No"
Then when you want to display or print one of the certificates issied for a student, click on the activity, then click on "View xx issued certificates" in the upper right corner (xx is the number of issued certificates).
You will then see a table with link to each of the issued certificates as a pdf file
And you will be able to download the certificate you want.
If "Save certificates" was not set to "Yes" the only way to get a certificate for a student is to use the "login as" feature of Moodle (see Dale's answer)