can I use non sashimi grade ahi tuna?

Sure. The recipe says that the sashimi grade fish is preferable (which implies that it is not recommended).

Assuming your tuna is super fresh, the biggest difference between the sashimi-grade cut and the normal cut will be the texture for this recipe. Particularly, the sashimi-grade cut will be from parts of the flesh that has less fibers. When raw, these fibers end up being like chewy string but cooked are softer.

You can counteract this stringiness a bit by chopping up the raw tuna finer than what is pictured in the photo. Thus, you will sacrifice some of the charm of biting into a generous chunk of tuna in exchange for less stringiness and a smaller dent in your wallet.