Roto brush

Hey

I was trying to use Roto brush to remove background from 3 seconds part of whole video but when I tried to double click on my 3 seconds clip nothing happened and new layer where I can use roto brush wasn't crated like it was in tutorials on YouTube. Double clicking on whole video (3 min) worked indeed. 
I know I can trim exact part from this whole raw video, but it will take a lot of time every time just to find part you need.

so do you have any suggestions how I can remove background just from this 3 seconds part? 
I attached video for better understanding 

thank you