If you can’t squat then don’t squat. Do not do stupid enabling stuff that looks like a similar movement but does not solve the root of the problem. One of the biggest problems I see today with virtually every athlete under 30 in the inability to squat without their heels coming off the ground. Much of this comes from the amount of sitting they do and the dress shoes they wear with elevated heels. Doing a so called sissy squat with the heels elevated will enable you to squat but does nothing to address the lack of mobility at the ankle manifested as a lack of functional dorsiflexion. If they are so tight that they can’t squat then do more three dimensional stretching of the calves, more backward running and walking. You will have to do lunges and step–up for strengthening until they gain that mobility at the ankle. Then when you begin to squat, start with bodyweight and progress incrementally to external loading.