As Bam said, harnesses are not good for many rabbits. Very few will tolerate them, and even less will actually like them. There are also a ton of things that can go wrong while wearing a harness like twisting their backs, arms/legs, neck, ect if something goes wrong. Its not recommended to take them for walks as if something scares them, they are at risk of severely hurting themselves
That said, i was able to harness train one of my rabbits. I did this because he is my emotional support animal, and being able to wear a harness is part of the requirements unless i plan to have his crated wherever i take him. He always has his crate in the car so hes not on the harness the entire time. He also wears his harness in his light carrying bag (with ice packs wrapped in towels) that i carry around sometimes. The only time he is on the ground in his harness is if he has to go to the bathroom while he is out with me, and he is allowed on the floor in some buildings. Dont pull; the rabbit has to lead you, not the other way around. If you tug the leash even a little bit too hard, the harness can cause injury and also scare the rabbit
As Bam said, too, you have to work slowly. I started with leaving the harness on the ground for a few days. Then putting it on him and giving him a treat, then taking it off as soon as hes done eating the treat. After he got used to that, we moved onto leaving it on a bit longer before taking it off. Once he was fine with it around the room, we added the leash on the floor and started in a similar way. Left the leash on the ground for a few days. Clipped it to him, treat, clip it off. After that, clip it to him and let him run around with it (letting him drag it around). After he was used to that, i would hold the leash and let him lead me around the living room. Being outside on hardness and leash still makes him very nervous, but he knows he can always be held or hop into his bag at any time
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