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 them. 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 friend does, it is probably because your profile does not match the assessment requirements yet.
The assessment is temporarily offline
Sometimes we have 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 may still 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The assessment is outdated and has been removed completely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 don't need them any longer and remove them completely. But you may still stumble across information about one of our old assessments somewhere on the Internet for a long time.
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 has been 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.