First, check fit and comfort. A harness should let your dog move freely and should not slip off. Some dogs dislike having something pulled over their head, so a different design may help. If this fear is new, handling seems painful or your dog snaps when touched, ask your vet before turning it into a training project.
Begin before the buckle
Place the harness on the floor while your dog is free to move away. Toss a treat nearby and let them investigate if they want to. End the session while they are still comfortable. Next time, you might simply hold the harness and reward your dog for staying relaxed. Only later offer a treat through the opening. Don't pull the harness over their head or lure them into a step they then cannot leave.
Watch for the small changes: turning away, backing up, stiffening or refusing a treat. Those are reasons to make the next session easier, not signs to keep the harness on all day until they 'get used to it.' The clips can be a separate lesson too: make the sound away from the dog's body before fastening anything.
Once your dog is comfortable wearing it indoors, let them move and play for a moment before trying a quiet outdoor step. If the street itself is overwhelming, work on that separately. A dog can learn to like a harness and still need help with a noisy walk.
Tonight, put the harness on the floor for a short, no-pressure look. Reward any voluntary interest, then put it away before your dog worries. If they won't approach, make the next attempt farther away or ask a qualified reward-based professional for help.
The AI trainer can help you separate gear fear from what happens outside, so the first step fits the part your dog is struggling with. For more on your timing during easy practice, see the dog-handling and training guide, first check fit and pain and let your dog move away from the harness
Sources: Dogs Trust on harness training. AVSAB on reward-based training.