Question: could not use the brush because the scratch disks are full?
Answer: If Photoshop 2019, or earlier, cannot launch because the scratch disk is full, hold down the Cmd + Option keys (macOS) or Ctrl + Alt keys (Windows) during launch to set a new scratch disk. ... Choose Edit > Preferences > Scratch Disks (Windows) or Photoshop > Preferences > Scratch Disks (Mac).