If you are having errors when uploading XML file for T4 Slips from QBO make sure that under Payroll Settings - Federal Tax your REPID info is filled in.
A better solution is to download the XML to Taxcycle and to file from Taxcycle
A client normally files their own T4’s but was having problems doing it from QB this year. I had the bookkeeper export the T4’s to an XML file from QB. I imported the XML into a new T4 file and filed the T4’s within minute. I was shocked how simple it was. Old dog, new trick.