Run United is one of the most attended and popular run event series in the countries and this weekend the 21km slots have been sold out. 5,000 slots for 21km are now taken and the management decided not to add new slots to ensure the safety of the runners and the smooth organization of the event.
If (at the beginning of the year) you decided to complete the trilogy, you had to have done your due diligence and register early.
Here's the official announcement from Run Rio and Unilab Active Health:
Note: Lower distances are still available but based on history it won't last long so do what you have to do.