The Select Topic To Improve (STI) feature is available only when the primary exam is set to JEE Main. However, the feature is useful for all the exams.
So, if you have applied for JEE Advanced or BITSAT, then you need to change the primary exam to be able to access the STI.
Once you have changed the primary exam to JEE Main, go to the Assessment screen of RANKUP.
You will be able to access the Select Topic To Improve from here.