If you are making financial projections, and you want to extend the planning period, you can do that any time you want.
Here is how to do it - click on the idea picture at the bottom left corner of the screen and you will open Idea Settings.
There, you will see the planning period that is set. In case you have selected the start of planning to be January 2021, and you set the planning period to 1 year, you will not be able to select any date beyond December 2021.
So, in order to extend the planning period, you will have to change the value to "2 Years" or more. The maximum duration of the planning period is 5 years.