This isn't actually strictly true. There's a tricky bit of math involved here, the idea of "certainty". If something is "certain" then we know that it will definitely happen. If something is "not certain" then there is a chance it won't happen, even if that chance is very small. If something is "almost certain" then we know that something will happen if we try infinitely many times.

If I flip a coin once, then I am not certain if it will land on heads at least once.

If I flip a coin a very very large number of times (like a billion) I am still not certain that it will land on heads at least once. That is because there is still a chance that I flip all-tails.

I I flip a coin an infinite number of times then I am almost certain that it will land on heads at least once. This is because in an infinite number of coin flips, the chance of all-tails becomes zero. We are "almost certain" that we will eventually flip a heads, but we are not "certain".

But here's the catch: it is not possible in reality to flip a coin infinitely many times, therefore in reality there is no way for a 100% chance of all-tails to happen. It can get very very very close to 100% but it will never be 100%.