How to make a certification manager nominated only?

Hi,

Thank you for reaching out to us.

We can restrict the certifications from self-enrollment through catalogs. That catalog should not be shared with users so that users cannot enroll themselves and in the meanwhile admins can enroll the users and that cert will be visible in their my learning list. 
Unlike learning paths & courses, we cannot define the enrollment type for cert. However, we can restrict the visibility of the certifications through catalog. 

I hope this helps. In case, you have any further queries, please feel free to write back to us.