If you don't have pistachios with no shells, just unshell them yourself. It'll only take 5 minutes and it isn't that hard.
Take basil leaves and wash them properly by keeping them in a strainer and running water water over them. Then use a clean dry cloth and pat them lightly to soak all the water from the leaves, and keep them aside.
Now, remove the skin of the garlic by pressing it or peeling it with a knife. While you're at it, shred some parmesan cheese too.
In a food processor, add basil, pistachios, grated cheese, garlic, olive oil, salt, black pepper powder, and lemon juice.
Blitz them together into a thick textured pesto, store it in a jar, and place it in the refrigerator.
Your pistachio sauce is ready!