The Rank Tracker allows you to keep track of the ranking movements for specific keywords. You will be able to see historical ranking data for these keywords, but history goes as far back as the date the project was created.
As a result, if you have just created a project or created it less than 7 days ago, you won't have the ability to select from the pre-set periods for that project. These options will be grayed out, as seen in image below:
So as time goes on, you will be able to select from these options.
If you have added your project over 7 days and these options are still grayed out, please contact support at firstname.lastname@example.org or via live chat.