"Although I've done everything to try and clear it"
Is your scratch disk on your hard drive or an external drive? How much free space is on it? You will need to clear space or use a drive dedicated for the scratch disk. Hold down hold down the Cmd + Option keys (macOS) or Ctrl + Alt keys (Windows) during launch to set a new scratch disk.
If the only storage you have is one internal volume, then it’s normal that you won’t have any other options to create a scratch disk. The option is available only when you have multiple volumes connected, like a desktop with multiple hard drives inside, or a laptop or desktop with external storage attached. (For example, when I had this problem with my MacBook Pro, I attached a fast external drive that was empty, and assigned that as an additional Photoshop scratch disk.)
Jane-e asked how much free space is on the internal storage of the computer you’re using; did you answer that question? The answer is important; depending on the size of the Photoshop file, many users need more free space than they think. Do you have over 50GB or so of unused space left?