Since no one here has ever traveled at the speed of light, How can we say for certain if it is possible or not?

Any mouse has hit on a key point in science. We can never say we're 100% certain about anything. Science has been proven wrong too many times (e.g., the flat earth or the geocentric universe). So what we do to account for that uncertainty is specify a degree of certainty in what we aver. So you might see a statement like "with 99.999% certainty we specify that mass cannot reach light speed." But there is that .0001 % of doubt still lingering. [I just made up the numbers for illustration.]

We can't say for certain that it's not possible.

But what we we can say is that there is an overwhelming amount of eviddence (from over 100 years of experimental testing) that Special Relativity is accurate. Amongst many other things, Special Relativity explains why an object with non-zero rest mass can't travel at the speed of light (an infinite amount of energy would be needed to get the object to light speed).

light is the only thing that has traveled at teh speed of light SO FAR