What were the structural errors of the League of Nations during World War II?

The League of Nations had ceased to be an effective organization long before the outbreak of World War II, a conflict it had singularly failed to stop. Its main weakness was a notable lack of authority, which meant that it did not possess any reliable means of enforcing its decisions. Furthermore, the League had been fatally undermined from the outset by the vote of the United States Senate in 1920 not to join. Without the formal backing of the United States, there was never any realistic chance that the League would accomplish its far-reaching aims.
At the same time, the League was fatally hobbled by certain structural defects. For one thing, it was set up so that any decision had to be reached unanimously by the executive council. In practice, this rarely happened as the signatory nations often had different interests relating to the crucial geopolitical issues of the day. The League of Nations Assembly was also chronically slow at reaching its own decisions, making effective action extremely difficult. And even when the League did finally act, it was unable to enforce its expressed will as it lacked a military force of its own. This meant that the League was totally reliant on the great powers to make good the omission, and for a variety of reasons, this was something they were extremely reluctant to do.
https://history.state.gov/milestones/1914-1920/league