Even if you pass all of your compulsory subjects, you will not have enough credits to complete your studies unless you also pass some number of optional (eligible) subjects. 

You can choose suitable subjects based on your interests and your timetable. You should take 1–2 electives each year in order to meet this requirement and also have enough time to pass your compulsory subjects.

You can register yearly maximally 9 credits from eligible subjects together for winter and summer semester for one year. (It is set by Code of study and examination of Charles University, article 4, point 10; calculation is 0,15*60=9)

https://www.lf1.cuni.cz/document/45284/uken-121-version1-custudyexam7thcode.pdf

If you will register more that 9 credits you have to know that these credits won't be credited to you. Also it is not possible to divide credits (for example you already got 8 credits, another subject which you registered has 3 credits and you would like to take just 1 credit from it, it is not possible, you won't get any credit for this latest registered subject).   

DO NOT FORGET to sign up for your  eligible subjects on SIS. It is your job to sign up for your subjects. The dates of the registration are always announced before the start of winter and summer semester on the faculty website.