Your dog used to stand for the harness. Now they turn their head away, back into the hall, or fold onto the floor when you reach for it.
This is worth investigating rather than pushing through. A dog avoiding one piece of equipment is telling you something about that equipment.
Safety note
Pain frequently presents as a change in behaviour rather than as obvious lameness. If your dog flinches, yelps or pulls away when you touch their neck, shoulders, ribs or legs, book a veterinary examination before you train anything. Get help now if they have growled, snapped or bitten while being harnessed.
Then check the harness itself
Look at the skin behind the front legs and along the chest strap. A red patch or a bare line is your answer.
Check the fit against the dog you have now, not the dog you had at 6 months old. Adolescents outgrow a harness quietly.
Watch how it goes on. A harness that comes over the head, or needs a leg lifted into it, is harder for a dog to accept.
Slide 2 fingers flat under every strap. If they will not go, the harness is too tight.
Re-introduce it with your dog able to leave
Consent here means your dog can walk away, and that you stop when they do. A dog who is held for the harness learns to leave the room sooner.
Put the harness on the floor, feed your dog near it, and put it away again. 30 seconds, twice a day.
Hold it up with the opening toward your dog and wait. When they move their nose toward it, feed and lower it.
Feed for putting their own head through the opening, with no buckle done up. Several days here is ordinary.
Do up one buckle, feed, and take it straight off. Only after that do you start walking in it again.
Working below the threshold at which your dog reacts is the condition for learning. Ducking away is the threshold, so the step before it is your session.
Keep the food out of the struggle
Food used to get a harness onto an unwilling dog teaches them that the treat predicts the thing they were avoiding. Then the treat stops working.
Do not lure your dog into the harness with food in your other hand.
Feed after a step your dog completed calmly, never during the part they are trying to avoid.
If you fed and the harness went on against their wishes anyway, that was a rep in the wrong direction. Go back a step.
Reward-based methods are what is recommended for training and behaviour change, and punishment-based approaches are associated with fear and avoidance. Harnessing is training.
You still have to get out of the door today
Use a flat collar and leash on quiet routes for a few days, if your dog accepts that without a struggle and does not pull hard.
Harness up in the hall where your dog is calmest, rather than at the front door with a leash already in your hand.
Shorten the outing to the pavement or the stairwell, and cover the rest with sniffing and chewing indoors.
Take a day off harnessing entirely if yesterday was a fight. A missed walk costs less than another bad rep.
Behaviour change is built from management plus reinforcement, and the collar days are the management half.
Today's version, in one line: check the fit and the skin underneath, then put the harness on the floor and feed.