Obama can reschedule his trip to Argentina in order to select another travel date. The change of date would allow Obama to pick a time that is not the 40th anniversary of the coup of 1976. It would still keep good relations with Argentina to plan the trip and avoid the appearance of any conflict.
A visit by the US president to Argentina on the 24th March would be perceived by many as a celebratory visit of the military coup that saw the junta take control of Argentina. This coup and the atrocities that followed were funded primarily by the United States. Leaked information shows that Henry Kissinger knew that the money would be used to fund human rights abuses. Therefore, it would appropriate to give the impression that this was something to be celebrated.
No, Obama shouldn't skip his planned trip to Argentina because this trip helps bolster our relationship with the Argentinean government and helps uphold the belief that the President of the United States is the leader of the free world. Obama's second term is about to come to an end, and it's important for him to keep these relationships with other nations intact for America's next president.
Barack Obama had nothing to do with the military coup in Argentina in 1976. His visit to Argentina on the anniversary of that coup should not be problematic. However, it's understandable that Argentine people would be upset about the United States' involvement in that coup, so it would be best for Obama to come on a different day. There's certainly no reason to skip the trip altogether.