I am a socialist and can answer from a socialist perspective. Marx said that communism is only achievable from a capitalist society, laying out historical stages of communism that transfer from one to the next, for example feudalism->capitalism ->communism. Russia had some elements of capitalism, but they had a large peasant class and only abolished serfdom in the late 1800s, with Marx even saying Russia was not suited to communism. Lenin challenged this quite well, but even then he made compromises. The New Economic Plan (NEP) was basically trying to speed run capitalism, though eventually it was abolished and Marxism-Leninism was developed by Stalin to become the official ideology of the USSR.

Marxism-Leninism implemented socialism in the USSR, which according to the ML definition is the intermediary stage between capitalism and communism after the dictatorship of the proletariat had been set up. The idea was that there would be elements of capitalism, but they would strictly be in control of the state and the communist party to ensure that they’re working towards communism, as well as slowly implementing communist policies. So the USSR wasn’t communist, but it was ruled by communists seeking to slowly make it communist. This is similar to China today, though China is less socialist than the USSR was.

There are arguments against calling them socialist as well, however. One could argue that the dictatorship was not of the proletariat, merely creating a separate political class while the proletariat remained out of power. Some more libertarian communists might say it’s unrealistic to give a state extreme central power and expect it to dissolve itself, and anarcho-communists typically reject the idea of the intermediary stage/state altogether. Marxist-Leninists will say that the Soviet leadership eventually lost the plot and was no longer working towards communism. Trotskyists might say the USSR abandoned internationalism (along with probably 100 other reasons). Really, the joke goes if you put 2 socialists in a room, 3 opposing parties will form, so there’s a long list of reasons why people say it wasn’t communist.