The mokutil package is used to update the MOK list with a certificate or hash. Mokutil package is not available for CentOS/RHEL 6. It is available for CentOS/RHEL 7 and 8.
If you are using CentOS/RHEL 8, make sure you have the CentOS/RHEL 8 (x86_64) BaseOS Latest repository enabled and then run command below to install the mokutil package:
# dnf install mokutil
If you are using CentOS/RHEL 7, also, make sure that you have the CentOS/RHEL 7 (x86_64) Latest repository enabled and then run the command below to install the mokutil package:
# yum install mokutil