I have an enclosed checkout flow that I want to allow users to go back one step at a time while also allowing them the freedom to jump to specific steps using the navigation. Is it a bad practice to place them close together?
I would advise against this. Unless you place enough spacing between the two buttons, you will deal with users accidentally tapping the wrong button. Have you considered moving the back button to the lower left section on the screen? It's hard to tell with just half your wireframe, but I think separating them one on top and one on bottom would help reduce user error while keeping both features.
This looks like a mobile design so I answer it with touch control in mind. Of course you have to take into account people who have big fingers. But mobile users often don't sit still either. They can be walking or simply changing their sitting position. This makes them less accurate in their movement and in hitting too precise a spot on their screen. Nielsen Norman Group has an article about it which says:
To avoid accidental taps, targets must first be big enough, and then also spaced well enough.
To answer the question directly: Purely from visual judgement I would think the two buttons are too close together. How big they should be or how much space is needed to solve the problem is hard to answer without more context. But I would start by reading that article and do some hallway testing amongst colleagues.