encountered an issue on a flight today from MUCL-TTPP My route of flight had me going to a VOR (POS) which is roughly 8 miles south of the field. I changed my route in flight to proceed to the Approach fix for the ILS rwy10 in TTPP. my airway in was the UL337 and so i made the approach fix (SHARK) be put into the CDU after VUDAL. This came up with an error on the CDU saying "VNAV Disconnect" and put the aircraft into "ALT Hold" The fixes on the approach had altitude restraints (SHARK at 4100ft and the TRINB at 1400) and the FMC didnt want to recalculate my descent profile. I removed the altitude restrictions to see if that would clear the error but that didnt do it either. So I had to go back to the old school way to calculate my descnet. I am wondering if anybody has had this issue yet or is there something I am doing wrong?