photoshop issue

And how much free space do you actually have? Post a screenshot of Preferences > Scratch Disks.

I'm trying to learn how to make youtube thumbnails so 1280 pixels by 720 is what I'm doing 

Did you check the things I suggested? What pixel sizes are you working with?

thank you for responding, so I also have premiere pro installed on my macbook pro, but I have friends that do photoshop on mabook air and it works better than on my pro one, what I'm trying to get at is, what do I need to do to make photoshop work on my macbook ?

The scratch disk is Photoshop temporary working data. Pixel-based image editing requires huge amounts of memory, much more than any RAM you may have installed. So it's written to disk, and by default it goes to your system drive.

Scratch files can quickly grow very large. Normally a 500 GB drive should be sufficient - as long as you keep only the operating system and applications there! Anything else, all your image files, should be stored elsewhere and not clutter up the system drive.

A normal configuration of OS and applications will take up around 90 -120 GB. Anything more and you need to do some housecleaning.

By the way it's also a common mistake to try to edit photos by 1. Create a new document. 2. Add the photos. No. Can cause bad issues if you don't know the app well. As a beginner use Open not New to start editing. 

If it says the scratch disk is full, then your disk is certainly full. A common cause is a mistake in image size. For example, instead of 2000 x 1500 pixels, you might have asked for 2000 x 1500 inches. Check this first.