It’s a priority for every cat to have her own territory. By scratching the wall, your cat is leaving her scent in a part of your home which she wants to mark as her territory.
Scratching the wall might be a cat’s way of relieving stress. There are many stress triggers for felines, such as changes in environment, a poor human-cat relationship, inter-cat conflict, etc.
Cats need a certain level of stimulation, both physical and mental, to feel motivated and happy. If your cat lacks this, she might get bored and will show this in some unwanted behaviors, like scratching your wall.