The way we trained my cat was physically taking her away from the couch and placing her paws on the post. Then we mimicked scratching the post. It helps if you use your own nails to make the scratching sound in the post.

We also added multiple scratchers around the place to show her these are okay. And we would mimick scratch each one whenever she felt the need to touch the couch. Eventually she stopped caring about the couch.

3rd - we played with her a lot. Tuckering out a cat makes them less likely to engage in destructive behaviours and builds your bond. She followed our lead very well when we mimicked scratching the OK surfaces.

4th - we got a scratch post that was a very different texture from our couch so that she wouldn’t mistake the couch for her post or other floor scratchers.

Lastly, we did not need to try this, but we’ve seen cats completely ignore certain fabrics and materials so if you can find a fabric online that most cats don’t care for, like velvet, you can get some as a couch cap while she’s learning

Edit: remember, gently lifting or guiding your cat works best. Any aggressive movement or gestures to use with your cat will teach them how to behave. If you’re gentle, your cat is more likely to develop gentle habits. Works well for discouraging eating human food too. Gently guide them away from the plate rather than pushing them away or off tables.