Urgent, can someone please help .

United agent check in counter at SFO can not print out my boarding pass for flight section PNH - BKK . And she does not know why . She told me that Changi airport transit staff will do it .
I arrived at Changi airport and transit desk agent can not print boarding pass either . No one to know why . I try to check in online and get error message " WARNING: Online Check-in Not Available For This Booking " .
There is also the note next to my name " Not available for Self-Service Check-In " . Does anyone know the reason . Our flight itinerary allows only 1 hr for connection flight . I do not know if can not self check in, I will not able to have a boarding pass and will miss our flight to Bangkok with Thai airways .

Have anyone having the same problem as myself have encountered so far . II wonder if that has something to do due to a wheel chair request noted on file for my elder , which prevents me to do self check in to print boarding pass .

Thank you .