Are you using the TeleportToPrivateServer function of the TeleportService? Not sure this is your issue, but could it be the case that you are providing a string to the placeId argument and/or a number to the reservedServerAccessCode argument?
The placeId argument must be an int64 (number type should be just fine) and reservedServerAccessCode should be a string. Just thinking this might have something to do with the error because you are loading the values from a data store. If you need to convert to either a number or string you can use tonumber() or tostring() respectively.