Please keep in mind that a visa letter might not be necessary for your particular case.
Please check with the Brazilian embassy or consulate closest to you for the proper visa process you must follow.
Also keep in mind that
Tourist/visit visas can be granted for those travelling for tourism, business, transit, artistic or sports activities, study, volunteer work, or to attend to conferences, seminars or meetings, among other purposes: a Business visa is not necessary to attend FAccT.
For further questions, you can reach out to the FAccT 24 General Chairs on the e-mail:
general-chairs [at] facctconference.org