How would I create a sidequest in UPBGE? An example is when the player must go to a specific level at a specific time of day in the game and shoot targets, etc and when surpassing the score or something like that, the quest is marked as ‘Quest Completed’. If the timer runs out, the quest is marked as ‘Quest Failed’.
This would be separate from the main ‘story’. This is seen in many games.
First you need to learn how to make an Interface. There are some tutorials on youtube, and if you want something more professional you can try bgui: https://upbge.org/docs/latest/api/bgui.html
Then, for simplicity, I would create an empties where, the quests should be. The empty would have the properties: Quest name, Quest description, QuestTimeLimit, QuestRunning, QuestAvailable.
Each empty should have a logic written in python associated with whatever the quest is available and if it was completed
All the info of the quest should be displayed in the the Interface when the player is near it.
Your question is too generic, and the solution might be a bit complex. You should try to make one thing at a time a when you know everything you need, you merge it all in a big project. Good luck!