Cors error to return bytes of file

[cyccro]

I'm trying to make a personal project which has to receive 2 folders and merge them and return a zip file. The issue comes that the code is working perfectly but it simply does not send the response. When i try to see the response, not even errors(the ones i had to return caused by some issue) aren't sent to the client side. The unique one sent is related to CORS:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:8080/merge_resource. (Reason: CORS request did not succeed). Status code: (null).

TypeError: NetworkError when attempting to fetch resource. <- sent by console.log

I already tried to search on internet but didn't find any solution for my problem. It might be a simple problem but me by myself can't find a solution for it.

Browser used: Firefox 127.0
OS: Linux Mint 21.3