My users are unable to add printers via QuickCode

Firstly, confirm that the users trying to add printers via QuickCodes are within your G Suite Organisation and they are a known Google Account. 

If they're not a known Google Account and you would like to allow these types of users to add printers via quick code then navigate to https://directprint.io/admin/quickCodeSecurity and toggle the switch for 'Restrict QuickCode access to known Google Accounts only'