Actually, if you want to solve the full scratch disk problem, you would need to delete temp files and clear the disk space. Some people may assign a new scratch disk(change a hard drive you want and set it as your scratch disk) to fix the error, but you are not recommended to do so as the problem has not been actually solved. And the other problem is, when you assign a new scratch disk, it can also get full after some times using. And for some Mac users, there is only one hard drive in it. In this way, please check the following two methods to solve the problem correctly.
Method 1. Free Up Scratch Disk Manually
Method 2. Clear Scratch Disk with Cleaner