 Complete the following phrase:  During this period, the United Provinces of the Rio de la Plata lacked a head of state, since the unitarian defeat at the Battle of Cepeda had ended the authority of the Supreme Directors and the 1819 Constitution. There was a new attempt in 1826 to write a constitution, leading to the designation of Bernardino Rivadavia as President of Argentina, but it was rejected by the provinces. Rivadavia resigned due to the poor management at the 