Thank you for your replies, i contacted Lufthansa again and they told me that as a result that their online check-in system is not in the agreement with UA(that is what i was told)i will have to do the check-in in the LH website for the LH segments and with UA the other segments. even though i will try to do the check-in first with UA and if i get any problem then i will do the check-in in the LH website.