A set of data will not always have a mode.
A mode is defined as the value that appears the most often in a set.
For instance, a set of 1, 2, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 6 would have a mode of 4, as it appears more often than any other number in the set.
However, a set of 1, 2, 3, 4, 5, 6 has no mode as there is no number that appears more than other numbers in the set.