What can I do to protect my walls from scratching dogs?

You can use a thin sheet of perspex or you can buy decorative wall paneling from a hardware store to cover it.

You can glue FRP (fiberglass reinforced panel) around the door. It comes in white bit can be painted. You can also use plastic laminate for countertops and glue that around the door.

You can repair any dog claw scratches with some spackling or dry wall mud and then sand smooth and repaint-prime first. You could mount a piece of plexiglass in this area so the dog would scratch that rather than the wall. I am assuming he/she does this when it's time to go outside. My dogs would always claw the wood trim too! You might be able to teach your dog to ring a bell to go outside! It's rather easy to do! Then you could mount the bell on the wall on a hook that is away from the wall. It is hard to hear the bell sometimes if you are not in the same room/area.

https://www.akc.org/expert-advice/training/teach-pup-ring-bell-to-go-outside

https://www.rover.com/blog/bell-training-dogs-really-works-heres

You can do the same thing with hot sauce. I had a puppy once that loved to chew the leg (just one) of my antique coffee table. I painted the sauce on the leg and he never touched a wood object again!