I've found this Buenos Aires city website offering free tours. I tried booking to confirm it's really free, and yes! It lets you choose the date and time, and it shows "$0," for example, for a tour in La Boca. I think it's the same for the bike tour. There's a WhatsApp number where you can inquire, and the website shows the entire itinerary. Hope this helps!
Recorrida Bicicleta Buenos Aires, ideal para conocer los mejores barrios de la ciudad con sus encantos! Prepara tu casco, energías y vamos a pedalear!
bafreetour.com
whatsapp number +54 9 11 6244-8039