How does a non-mobile-friendly site hurt my rankings?
Google is now penalising sites that are not mobile friendly and have been doing this since 2015. This quite simply means that not only are you leaving money on the table but you are damaging your ranking, business prospects and diminishing your chances of re-gaining lost ground.
What is the official Google criteria for being Mobile Friendly
Don't use outdated software like Flash which is not normally used on mobile devices.
Don't make users zoom in to read your text.
Make sure your screen layout adjusts to the portrait or landscape aspect ratio you're currently viewing.
Give your links plenty of room for people with fat fingers.