Mars has no magnetosphere so it’s very hard to have a breathable atmosphere.

I don’t know why this has become such a common talking point about why colonizing Mars is hard, it really has no significant impact.

For starters, it’s only meaningful for terraforming. Regular realistic colonization involves setting up domes or tunnels, none of that’s affected in any way by Mars’ magnetosphere or lack thereof.

As for terraforming, the lack of a magnetosphere means that Mars will “leak” atmospheric gasses due to solar wind sputtering over periods of time that are short on geological scales but are vastly longer than anything a human civilization will care about. If Mars were to magically have an Earthlike atmosphere appear on it today it’d be millions of years before it became unbreathable by this process. The human species has only existed for a tenth that long, and our civilization has only existed for a hundredth of that. If anyone still cares a million years ago they can just top the atmosphere back up again by whatever method they put it there in the first place.

Or, if you really have your heart set on that magnetosphere, build one.