The reason one cannot calculate time dilation with speeds greater than the speed of light is due to the fundamental principles of special relativity.
According to special relativity, the speed of light in a vacuum is an absolute constant, denoted by the symbol "c." The theory also states that no information, energy, or matter can travel faster than the speed of light. This limit on the speed of light is a fundamental feature of the theory and has been experimentally verified.
One of the consequences of special relativity is that as an object approaches the speed of light, its relative mass increases, and the amount of energy required to further accelerate it also increases. As an object with mass approaches the speed of light, its kinetic energy increases, and its momentum approaches infinity. Therefore, it would require an infinite amount of energy to accelerate an object with mass to the speed of light.
Given that time dilation is a consequence of the relative motion between observers, it is derived from the effects of Lorentz transformations, which involve velocities relative to the speed of light. These transformations only apply for velocities less than or equal to the speed of light. They do not provide meaningful predictions or calculations for velocities greater than the speed of light.
If an object were to travel faster than the speed of light, it would violate the principles of causality and lead to paradoxical situations. It would imply that an object could potentially arrive at a destination before it even started its journey according to a slower observer.
Therefore, due to the fundamental limitations imposed by special relativity and the constancy of the speed of light, time dilation calculations are not applicable for speeds greater than the speed of light.