This is a premium feature, available with Customify Pro.
To have this item working, you need to enable the Header and Footer Builder Booster module in Customify Options:
After activating a module click on Customize > Footer option to get started. Drag Icon Box item into a footer row to add it and then click to gear icon to open customize settings.
Every the Footer Icon Box feature work the same the Icon Box item in the header. You can refer Header Icon Box (PRO)