There is a quest you get from Dranos inside the club in Nibiria. The quest will allow you to access the sewer entrance inside the same building where the quest starts.

There are a few ways to obtain a medallion:
1): after talking with the Academy Officer there's an encounter outside with 3 Sith Students who will drop medallions when killed
2): Inside a chest on the top floor of the Zukar Building. Located in the penthouse behind the hackable door.
3): Inside a chest on the top floor of the Apartment Building. Located in the room behind the hackable door.

I hope this helps!

Also, is there any way to buy upgrade modifications? I want to buy some because I have 50 levels of eperiance and nothing to spend on

Hello,
1: After checking I found a command block that was set wrong, preventing the reward + removal of the taskpad. Thank you for bringing this to my attention! If you still want your reward, you can use this command once: /give @a minecraft:ghast_tear 25

2: I don't believe anyone on Tatooine sells biotoxin. However you may find more from looting chests & brewing stands. You are free to leave Tatooine & come back anytime with more to finish it. You might find a seller on a different planet.

3: Yes. Gaining dark side points over light will make you unable to enter 'light side areas' like that in the basement. Gain enough dark side points & you'll be allowed to enter 'dark side areas'. Once you lean far enough towards one side, you will be able to access their areas + powers. Choosing both sides isn't recommended as you will gain nothing. You are able to check your dark or light status in the visual crafting index from the galaxy map.

I hope this is helpful, thanks for the feedback + support!