Why my saké turned out sour?

Ive had some of the same experiences with Sake. my first batch was so delicious, the next one..not as much. It seems to me that a possible contributor to your situation is that the solution was allowed to sit undisturbed for too long. I have had batches of Sake turn when they were allowed to do their own thing for too long. As a brewer, I was more than a little surprised by the process differences from beer to Sake in that Sake needs a lot more gradual attention, in the form of stirring and additions of rice Ect.

Based upon your answer in the comments, I suppose that your sake has been contaminated by acetobacter. These bacteria convert ethanol into vinegar. That means you now have nice sake vinegar. As such it is probably usable in your kitchen after pasteurizing and somewhat diluting it.

Now as for the source of the contamination, you need to take into account proper cleaning and sanitizing practices throughout your process. Buy a cleaner like PBW, a no rinse sanitizer like Star San, and make sure that all your equipment is clean before you start, and that your fermentation vessel is sanitized before you add the ingredients to be fermented.

When you let things cool, cover them with a lid and possibly with an extra towel (over the lid), also to avoid contamination.

Hopefully your next brew succeeds again.