You lift the brush. Your dog ducks away. You could hold them in place and finish the job, but then the brush becomes a warning that their choice no longer matters.
First, look for discomfort. Mats, sore skin, ear problems, and pain can make ordinary handling unpleasant. If your dog growls, snaps, seems suddenly sensitive, or has severe matting, stop and speak with your veterinarian or a qualified groomer about a safe plan. Don't cut tight mats close to the skin yourself. A training exercise is not a substitute for needed medical or grooming care.
Start before the first stroke
Put the brush on the floor and reward your dog for noticing it at a comfortable distance. Then put it away. Next time, pick it up briefly, reward, and put it down. If your dog can choose to walk away, let them. That choice makes it easier to see when the step is too hard.
When they remain relaxed, touch an easy part of their body with your hand, reward, and stop. Later, try a soft brush for a single, gentle stroke in a place they already enjoy being touched. Reward and finish. The aim is not to brush the whole coat in one sitting. Build duration over several short sessions while watching for stillness, turning away, lip licking, or other signs that they need a break.
Choose a brush suited to the coat and use gentle pressure. If one body area remains sensitive, don't keep rehearsing it because other areas went well. Ask a professional to check that area. You can separate essential grooming from practice: a groomer or veterinarian may help with urgent coat care while you make future brushing less stressful.
A dog who walks over when you put the brush down has made more progress than one who stays still only because they cannot leave.
Sources: Dogs Trust: How to handle your dog. Dogs Trust: How to understand your dog's body language.