If you grant workers a qualification to take your study, they may receive a message informing them they are unqualified for a variety of reasons, some of which are described below.
Perhaps the most common reason workers will receive a message that says they are not qualified to take your study is that it often takes MTurk a few minutes to assign qualifications after you launch your study. If a worker tries to accept the HIT before the qualifications are assigned, they may receive a message saying they are not qualified.
Another reason workers may be told they do not qualify for your study is if you have demographic or worker qualification requirements that the worker does not meet. For example, you may grant a worker a qualification because they meet a demographic requirement for your study. However the worker may receive a message saying they are ineligible to accept your HIT if they do not meet other criteria for your study, like a 95% approval rating.
Finally, workers often report that they cannot accept a HIT when they have completed or accidentally accepted and returned the HIT previously. Mechanical Turk only allows workers to accept a HIT one time. Thus, if the worker has previously accepted your HIT, they will be prevented from doing so again.