Hi, my name is Tim and I'm 10 years old

I'm using SolidWorks  2017 and following a tutorial of a gearbox and I have to thicken this part and the thicken tab isn't high lighted.

Can I please have some help on this part?
Attached is my file with a part that needs to be thickened.

I would also please like an explanation on why the tab is not high lighted to thicken my part.

Thank you