Deadlifts are more taxing than squats, so as to not stress the body to a point where it could not recover in a useful period of time, the deadlift is limited to one set.
If you can’t squat and are using the deadlift as your primary driver of stress for the lower body I would do 3x5 until it gets heavy then cut back to one heavy set with two back off sets.
Perhaps more importantly, why no squatting?