Noun for "lack of exercise"

Solution 1:

A lifestyle devoid of physical activity is usually described as sedentary, as for example in this Wikipedia article on Sedentary Lifestyle.

Solution 2:

I think inactivity may suggest the idea:

• the state of being inactive , idle or inert.

Source: www.thefreedictionary.com

UNHEALTHY DIETS & PHYSICAL INACTIVITY

• Unhealthy diets (especially those which have a high content in fats, free sugars and salt) and physical inactivity are among some of the leading causes of noncommunicable diseases (NCDs) including cardiovascular diseases (CVD).

Source: www.who.int/nmh/publications (World Health Organization)