How come I am unable to trim a portion of a circle.

Hi I am currently trying to follow this Youtube video. But I am unable to do the step when trying to trim the circle. The error I get is, "Cannot used a closed entity when trimming". Can someone please tell me how I can do this? I am completely new to solidworks. Thank you!

Creating Spur and Helical Gears - Tutorial - SOLIDWORKS - YouTube 

Step I am trying to do:

My drawing:

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