When can I access the friends list in Mario Kart Tour?

I've been playing Mario Kart tour, and got a notification that someone sent me a friend request!

But when I try to check my friends list I get the message "Unlocked when ranking is available."

I think I've played long enough for most of the training wheels to come off, but how long do I have to play to use my friends list?

You need to unlock the fourth or fifth cup in the list (depending on the week) in order to unlock ranking. Once you've done that for the first time, you will be able to access the friends list and see how you compare vs your friends.

(addendum: week 1 ranking was on cup 4: Koopa Troopa. week 2 ranking was on cup 5: Toad. week 3 ranking is on cup 4: Bowser Jr. My assumption is that this pattern will hold in 2 week cycles)