The Photoshop scratch disk is your hard drive.
Photoshop uses your hard drive as temporary “swap” space, or virtual memory when your system does not have enough RAM to perform an operation. If you only have one hard drive or partition in your computer, then the scratch disk will be the drive where your operating system is installed (the C drive on a Windows system).

 if you want to remove this error please follow these steps.
Step 1:   Open photoshop cc 2017
Step 2:   Press Ket Ctrl+K
Step 3:   In prefrences->scratch Disk you can select multiple disk for space
Step 4:   after Selection press ok button
Step 5:   Your problem removed.