Allowing the SC Adjustments add-on gives your character access to those, but you still need to add the adjustments you want to the character.
When creating a character, once you have selected a 1st level class*, go to the Adjust tab (the Personal tab could also be used for this one). In "Other adjustments", click "Add a new adjustment". Find the "Drawbacks allowed" from the selections and select it. Set its incremented to +1.
*Before you select a class, you have no slots for feats (and similar things such as flaws, traits and drawbacks)
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