It's not sure what you mean by "Stalin's" foreign policy, or what you think where the successes/failures.

The USSR had two notable Foreign Ministers in the 1930s, Maxim Litvinov and Ribbentrop. Litvinov's Foreign policy was based on conciliation with the west, and isolating Germany. Litvinov successfully mended some fences with France and the UK, and secured diplomatic recognition from the USA. IIRC he was also responsible for the USSR's entry to the League of Nations. But I guess he wasn't that successful, given the Allied capitulation at Munich.

After Litvinov, the USSR changed tacks and put Molotov in the Foreign Ministry, whose most famous legacy was the nonaggression pact and the division of Eastern Europe. The post facto rationalization was that this was meant to secure breathing room for the coming war, which took Stalin by surprise anyway.

His western foreign policy was so bad to simplify it because several foreign western nations such as France, United Kingdom, United States, Japan, Australia, Czechoslovakia, ect intervened during the Russian Civil War supporting one of the Communists (Reds) enemies the (Whites). The Communist philosophy and system being firmly anti-capitalist, anti-imperialist, pro revolution is the antithesis to many of these western nations fundamentals. This is a very simplified and broad summary but hopefully it gives you a start.

The main reason is that after the Bolshevik revolution, the treaty of Brest-Litovsk (where Soviet Russia unilaterally pull out of WWI), the foreign intervention in the Russian Civil War), and with Communist ideology being anti-Capitlist, the western powers were very unwilling to work with the USSR, and that left Stalin with few options to pursue. He tried to form an anti-German coalition but that plan had only tepid response from the west. As was already mentioned, the Soviets weren't even invited to the Munich conference. Without any allies, and facing a imminent major war, Stalin had to do with what little options he had.

Also, Stalin's foreign policy looks bad with the benefit of hindsight however his decisions were fairly rational and pragmatic, given the choices and information he had at the time.

The Conflict: Stalin didn't respect the national borders created by Versailles. In the 1930s, the Western democracies wanted to maintain these borders to keep international peace. Stalin felt no need to protect the system, but ironically his opportunism frightened the Western fascist states, who felt threatened by the sheer size of the USSR and its obviously brutal policies towards preexisting elites.

Unjust borders Versailles and the post-WWI peace based its legitimacy upon the rights of the governed to self-determination. Many of the national borders created in 1918 were regarded by certain political factions as illegitimate, more reflections of opportunism amongst the victors over the defeated empires than of true justice. As heirs to the Russian Empire, a vast nation that had controlled 15 modern-states and hundreds of ethnic groups, Stalin and the Communists had no respect for an idea that would dismember their worker's state. They also had little respect for the sanctity of the borders of their immediate neighbors, who had basically seized independence during the Russian Civil War at the expense of the central government in Moscow.

Unnecessary borders Militant communism moreover has little desire to allow national borders to become breakwaters against the spread of communism. More pragmatic men like Stalin may not have been risk-takers willing to attack all the capitalist nations, but they felt no qualms about invading smaller neighbors like the Baltic nations and Finland. However, while these actions seemed reasonable to Stalin, they were regarded as militaristic aggression by the Western democracies, who shunned him for them.

Distrust: Eying each other as enemies, the democracies, fascists, and communists were heading towards war. Stalin likely thought the decline of democracy and its replacement by fascist powers was a fairly insignificant trend amongst equally evil capitalistic nations. Seeing all as enemies, he ignored the main fact: Germany was rearming, aggressive and eliminating the relative balance of powers in the interwar era with a centralized hegemony.

Stalin arguably should have realized that the greater danger lay from the fascist powers, but his Popular Front tactic was belated. Furthermore, he was a victim of bad luck. In Central Europe, he attacked the Polish dictatorship, reconquering land that had been taken from Moscow in the 1920s.

Underestimating German strength. Stalin's strategy was to let the democracies and fascist dictatorships fight it out in a war of attrition mirroring the Western Front of WWI. This backfired spectacularly. The French were broken quickly by blitzkrieg, leaving Stalin alone to face off against Hitler. Dealing with a dangerous situation, Stalin exacerbated it by failing to see that the international peace created by Versailles was to his benefit against the initially more industrialized German economy.