Reward milestones are locked by default and will unlock periodically over the course of each Pit Pass.

One milestone becomes available every 24 hours on weekdays and two milestones unlock on Saturdays and Sundays.

Milestones can be completed while being locked due to the time restriction but rewards cannot be claimed.

The collection of Trophies is in no way limited and you can collect as many as you like every day and continue to progress on the milestone track. The restriction only applies to claiming rewards.
Make sure you log in every day to maximise your earnings!

The time restriction applies to both Free Pit Pass and Premium Pit Pass rewards, and activating the Premium Pit Pass doesn’t make it disappear.