How do I add it to roll20? I can't manage to add the character sheet to the game. Can someone help me?
Try using https://miro.com/ You can drag and drop any materials and customize them there live.
Miro allows you to draw on the sheet, just like in real life (see video on editing hogwarts sheet here: https://imgur.com/sjqZVxL)
PbtA games don't require complicated automatization, since the character sheet editing is literally about marking and underlining a few dots and words :) For a character sheet to be available in Roll20 it needs to be created and exported there, since it's not available in options (you can check it by "create a game" and "choose character sheet" options in roll20) it's not been exported there. And, frankly, one does not need it :)