Before taking a task from the guild board, read its description. Pay attention to what you are asked to do and in what time frame. This can be harvesting crops, gathering resources from animals, preparing products in factories, opening decks, sending ships, serving travelers, helping friends, and more.
There is a strict amount of time allotted to complete a task. It counts down from the moment you take the task. Collected resources begin to count as well after you take the task. If you do not meet the conditions in the allotted time, the task will be considered a failure. You can only try to complete a new task, if you have not exhausted the limit of tasks in the current league.
By the way, after completing the conditions of the quests, do not forget to enter the guild building to get a point for completion. Here you will also be able to pick up a new task or reserve the next one. The booking button is next to the "Start" button.