There can be several reasons, why an assessment is not available for you.
You are not qualified for it
All assessments come with requirements the clickworker must fulfill in order to see it. Some require a particular native language skill, others may require a verified phone number, another assessment already passed or even experience in particular projects. If you don't see this assessment but your fried does, it is probably because your profile does not match the assessment requirements, yet.
The assessment is temporarily offline
Sometimes it is necessary to take down assessments for technical maintenance or other reasons. If that is the case, nobody is able to take them until we reactivate them. In such cases, we try to keep the downtime as short as possible, but it still may be several days or even weeks before the assessment returns to your workplace.
The assessment is outdated and has been removed completely
Remember the address research assessment? If you do, chances are good that you are a Clickworker for quite some time, because we stopped using it years ago. Assessments are not everlasting, sometimes we decide we just don't need it any longer and remove them completely. But you may still stumble across information about it somewhere on the internet for a long time after.
You have already taken the assessment in the past
A point easily overseen is the possibility that you have already taken it. Especially if your account is existing for quite a while, but you haven't really used it much in the past. Chances are good that you already took your chance and the assessment is one of those that are not repeatable.