Is it obligatory to make up a day of Ramadan fasting if a fasting person wakes up and has a sour taste in their mouth, believing that it returned from the stomach during sleep, knowing that they rinsed their mouth after waking up and removed the taste?

It is from Allah's mercy upon His servants that He does not hold them accountable for what is beyond their capacity. A sleeping person is not held accountable because his intellect is suspended. There is no sin upon someone from whom something exited while he was asleep and then returned to him, nor is he required to make up for it, especially if there is mere doubt about that happening.