Which Eastern European country lay within the Soviet sphere of influence but did not join the Warsaw Pact?
The answer to this is Yugoslavia. Yugoslavia was a country that was in Eastern Europe. It was a communist country and was within the area that was dominated by the Soviet Union. Even so, it was not part of the Warsaw Pact.
Yugoslavia was a communist country, but it was more resistant to Soviet influence than the other countries of Eastern Europe were. Marshal Tito, the Yugoslavian leader, was fiercely independent. He did not want to simply allow his country to be a satellite of the Soviet Union. Because he was so determined to maintain independence, he defied Soviet leadership and Yugoslavia was eventually thrown out of the Comintern in 1948. The Warsaw Pact was established in 1955. Since Yugoslavia had been ejected from the Comintern, it was not included in the Warsaw Pact.
Thus, the best answer to this question is Yugoslavia.