The "[structural] friction" is defined by the tweet, not the substack article responding to the tweet.

"I truly believe this lack of structural friction when it comes to basically every type of dopamine-frying pleasure on earth is a huge part of why gen z is Like That"

You consider that a definition?

It's context. A branch of a discussion thread. Not a dictionary.

If you want a faster (less friction?) answer, you could post your question to substack comment thread or twitter.

"Friction" in the author's post refers to intellectual friction. The need to think about what you are doing before you do it; as opposed to being led to your next action by the UX of an app or instruction of another person.