This is another targeted offer from Radisson Rewards. You should have received an email if you were invited for this promotion. You can get double elite qualifying stays and nights count for all your stay from September 3rd to November 30th, 2019. You need to sign up for the offer before you book your stay. Note that you don’t earn any extra points under this promotion. Only nights and stay count.

Here is the general requirement for various elite levels for Radisson Rewards:

Elite Status Requirement
Club 0 (default)
Silver 9 nights or 6 stays
Gold 30 nights or 20 stays
Platinum 60 nights or 30 stays

You can check the benefits of various levels here.