If someone experiences this problem, let me explain why this happens. After a candidate took an exam (first attempt), Mettl keeps the record of their name in the exam. Once a new attempt is used for registration, the functionality to share the link is triggered but no link is sent because the user's name already exists in the exam.
To solve this problem, a trainee needs to send an email to academy@mendix.com. The TOTW will manually remove registration of the candidate so that they can register again. This is for now a workaround untill Mettl changes their functionality on the back-end.
Good news! There is a hot-fix implemented and everyone who experiences the missing test link, need to register again, that would resolve the problem!