Error: Call to undefined function bccomp() in CRM_Financial_BAO_Payment::isPaymentCompletesContribution()
Apparently our PHP installation was missing the bcmath package which bccomp() needs. By installing the package via php7.1-bcmath we were able to then process the refunds accordingly. Did not see it here (https://docs.civicrm.org/sysadmin/en/latest/requirements/) but I could be missing something?