Says my scratches disks are full when I have 13gb free

The thing is you tried to save a Gif. I do not know it it was as an Animated gif or not. If it was a animated gif.  Save for web may have need scratch space to render full size frames for the animation. If the frame size is large and it has the max 500 frame that like 500 images  Photoshop may  have calculate  it need more them 12GB of scratch space for what it need to do and that was not available. I haves made mistakes in Photoshop like create a need document 800x800 and got that massage when I had 80GB of free space.  I did not notice the new document dialog was set to inches not pixels so 800"x800"  at 300ppi its like 80,000px by 80,000px. I do not know how Photoshop manages resources or request resources.  I do know  I have seen Photoshop use large quantity of free space for its scratch work.