When I try to remove researchers from a specific repository, I receive an error and am unable to do so. Why is this happening and what can be done about this?
GitLab repositories inherit membership and permissions along the group hierarchy structure. It is probably the case that the researcher you are attempting to remove is part of the group that contains the repository. Work your way up the hierarchy, checking for membership and permissions at each level.
Note that your organization might also use their enterprise authentication (like Microsoft Active Directory) for logging into GitLab. If so, they might be synchronizing user groups into GitLab, and then membership of groups in GitLab might really be controlled outside of GitLab itself. Your IT folks should be able to help you figure out where membership and permissions are set for groups in your instance of GitLab.