Most exams have no prerequisites — to become certified, you simply need to take and pass the relevant exam. For example, earning the LFCS requires only passing the LFCS exam, and earning the CKA requires only passing the CKA exam. Training courses are optional and not required for any certification.
The one exception is the CKS exam: candidates must have already achieved CKA certification before they can schedule it. The CKS exam can be purchased at any time, but scheduling is blocked until CKA certification is on file. That CKA certification doesn't need to be currently active, just achieved at some point.
So can I purchase the CKS exam if I haven't passed the CKA exam yet?
You may purchase the CKS exam if you haven't passed the CKA exam, however, you will not be able to schedule your CKS exam until you first pass the CKA.
Can I schedule or take the CKS exam if my CKA certification has expired?
The Certified Kubernetes Security Specialist (CKS) certification requires prior completion of the Certified Kubernetes Administrator (CKA) certification.
Your CKA certification can be expired - it simply needs to have been obtained at some point.