That's because you have selected, among others, Maastricht University's Faculty of Economics and Business when registering to OMPT.
Students who apply to Maastricht University's Faculty of Economics and Business will not be able to purchase the OMPT practice material, but can participate in the Maastricht University Math Summer Course instead. This contains practice material which is very similar to OMPT-A and OMPT-B practice material, and includes a teacher you can ask questions. It furthermore has additional practice exams, created by the teacher, which are automatically graded. The summer course is available throughout the year.
Because you can't purchase the official practice material, you also can't purchase the bundle.
We always recommend students to participate in the Maastricht University summer course due to the additional benefits. If you don't want to take the summer course and prefer to take the practice material instead, please contact OMPT support.