Methods of Proof in Mathematics

[Giantzebra]

I doubt you'll find a free course that gives you any credit, so I doubt they'd accept a non-credit course for meeting the prerequisite  

if you live in new york state the SUNY system (State University of New York -- a group of public universities) has a bunch of upper level math courses that can be done online -- probably too expensive if you're out of state though        
$1412 for 4 credits      
$353 per credit for online out-of-state residents

https://www.esc.edu/tuition-financial-ai...ion-rates/

list of courses      
https://open.suny.edu/courses/search?l=U...athematics

they even have                    
MATH 4010-01 - Abstract Algebra I: Group Theory
https://open.suny.edu/courses/empire-sta...-fall-2020

I think they might accept a non-credit option. Their logic is that students were failing without the pre-req.