How do I push changes to a downstream forked repository?

I have a collaborator who has forked a repository to work on a side project. The fork is here, and Upstream is here. I have made some changes on a ranch of the forked repository. I’m trying to push the changes to origin branch_name but am getting

error: failed to push some refs to ''

I don’t know if the problem is with my credentials, (working on Mac OSX) or if the forked architecture is not the right set up for this particular project.

I need to be able to assist my collaborator.

A forked repository belongs to the person who forked it, so by default no-one else can push there. If the collaborator wants to give you access, they can either add you as a collaborator to the fork or (more specifically, so safer) create a PR and enable the checkbox to let people with write access to the target push to the PR branch.