You can choose the courses for your APSP from the list of courses suggested in the curriculum of your degree programme. For compulsory courses you need to consider  the pre-requisites (courses that  have to be taken before others). This means that, in order to add certain courses to your APSP, it is necessary to have included the pre-requisites first.
Pre-requisites  are listed in the curriculum. Therefore, before you make your APSP, you should read the section related to your programme curriculum in this Student Guide.

For Bachelor's degree programmes: before you can instert new courses belonging to the next academic years, you must have inserted in the APSP all the courses belonging to previous years, except for free credits.

In  any case, the system running the operations prevents you from making an irregular APSP in which the pre-requisites are not respected. A  warning message tells you  which courses must be inserted before others.