User is trying to send a client to QuickBooks Online, but gets an error message that says the client already exists. The user is unable to send the client's projects as well.
This can happen if the client exists in QuickBooks Online, but is not mapped to the corresponding record in Core. You should manually map the client record in Core and QuickBooks Online in order to sync the projects successfully. Check the Core Help for details on that.
Another option is to set Core to overwrite client records if they already exist in QuickBooks Online from the QuickBooks Sync Detail > Sync Settings > Send screen. Check the Core Help for details on the If record exists, overwrite? field.