Having issues with wire cut on a mac

I am trying to create a pocket in this solid and I keep getting little explanation marks. I am NEW to using rhino for mac, I used wire cut in windows but can not get this to work. I am in Metric and want the larger opening to be a pocket .5 of the way through the block.
Thank you Neal Jr

Mortis_Tennon.3dm (478.2 KB)

Your larger slot curve isn’t a closed curve. Fix it and you’ll be able to use it.

I assume this is what you’re after

Here a quick recording of how I did it

Thank you! This is excatly what I was trying to do. Thank you Cheers!