"Can't fail" is not the same as "impossible".
Both indicate that something is important. But "can't fail" is an exhortation to the troops. A bit hyperbolic, maybe, but it's usually meant to excite and inspire. Note that it's also a bit of a cliche and often prompts the question, "yeah? what if we do fail?".
"Impossible" is demotivating. It just says that there's no way you think you can do this because everyone is just not that good in this scenario.
When you're attempting to motivate a team, neither of these is great. If you have to use one, though, say, "can't fail." Don't say "impossible".