The incorrectly spelled word is "trenquility." The correct spelling is "tranquility," which refers to a state of calmness or peacefulness.

Explanation of the other options:

☀ Vacuum: This word is spelled correctly. It refers to a space entirely devoid of matter or air.

☀ Acceleration: This word is spelled correctly. It refers to the rate of change of velocity of an object.

☀ Schedule: This word is spelled correctly. It refers to a plan or timetable of activities or events.