Why Can't We Live on Mars?
Humans cannot live on Mars because neither the temperature nor the atmosphere is compatible with human survival. There is also no edible food and very little available water. However, given the construction of an artificial environment, life on Mars could be possible.
The Martian atmosphere is 95-percent carbon dioxide. It is only 0.13-percent oxygen, far less than the 21 percent available on Earth. The atmosphere is also nearly 100 times thinner than Earth's, meaning Mars cannot retain the radiant heat of the sun, so while late afternoon at the Martian equator may be a comfortable 70 F, at night the temperature would fall to 100 F below zero.
Since humans have lived in space for extended periods of time in ships or orbital station, it's reasonable to suppose that life on Mars is also possible. This assumes that Earth-like atmosphere and temperatures, as well as food and water, can be provided, but providing such an environment on Mars would be enormously expensive. Even more costly would be the constant trips to Mars to resupply the colony. While it is possible that a self-sustaining colony could be designed and constructed sometime in the future, the technology does not currently exist to make a Martian colony feasible.