What does friendship level unlock?

Trading is not open with every friend. It waits until your Affinity with that player reaches a set level, and it is counted per friend: each friendship climbs on its own.

Reaching the trading level is not the end of it. Each higher level adds more Aniimo types to what you two can swap, so a friend you only just unlocked trading with can't take everything a long-standing partner can. If a trade you want isn't allowed yet, the fix is more Affinity with that player, not a different menu.

Gifts are the easiest way to raise Affinity. A Gift of Celebration is worth 99 affinity on its own, so if you are pushing one friendship toward a trade you have planned, gifting is where your effort should go.

Not settled: The exact Affinity level that opens trading isn't known, and neither is which Aniimo types each higher level adds.