Just downloaded and started the tutorial. Immediately got stuck because I cannot figure out where the "empty box to the left of the planet layer" is located.  The tutorial will not continue until I click it and I've tried clicking all over. I'm going for a walk now.

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If I understand you correctly it looks like you need to drag the tutorial over to the left side of the screen. Your layers will be underneath it. I am guessing the box will be there to maybe toggle the layer on and off. I could be wrong my copy of photoshop is old as dirt. I think I have CS6 but most features of the program have remained relatively unchanged over the years.

ETA: OK I read the instruction on your tutorial. You do need to drag it over to access your layers, and you will need to click the little square box  The left hand upper green circle in my photo below.

I am guessing it will be one of the 2 boxes I have cirled here depending on what you are doing in the tutorial

Thank you, that was it. I didn't think about a needed part of the screen being covered up.