Why was it difficult for the former Soviet Union to establish friendly relations with the European states during the 1920s?
I would say that one particular reason why it was difficult for Russia to establish appropriate relationships with nations in the 1920s was due to its own state of governance. Given the Revolutions that had taken place in 1917 to overthrow the Czar and the Provisional Government, there had been much in way of instability in Russia at the time. This made it difficult for alliances and negotiations to be formed with other nations because its own state was one in flux and in constant challenge. I think that this might contribute to the level of internal stress, making foreign alliances and policy nearly impossible to conduct. When nations are not clear of their own goals and ideas, communicating what it wants from other nations cannot effectively be executed.
The nations of Europe, especially Western Europe and the United States, had allied mostly with Czarist or "White" Russia during its Bolshevik Revolution and the civil war that followed, to the point of sending military aid to them. As the war dragged into 1924, the government was steadily losing to the Red Army. The bitterness over the Western alliance with the Czar made it difficult to establish relations. The new communist government also had its hands full consolidating its power and control and launching an entirely new system of government. Relations with the west were not the priority.