What to do when photoshop says that my scratch disk is full.

Please read this:

https://helpx.adobe.com/photoshop/kb/optimize-photoshop-cc-performance.html

What are your Scratch Disks (Photoshop > Preferences > Scratch Disks) and how much free space do they provide?

That message is usually very real, and you should take it very seriously. It means your system drive is so full that your whole system is about to choke.

First of all, check how much free space you have on your drive. If it's over 90% full, it's critical and you need to take action immediately. If it's over 70% full you should still do some housecleaning and remove everything not needed.

The problem with system drives is that all kinds of things accumulate there, and they fill up over time. To make matters worse, most of it is in your user account, folders that are hidden by default, so you don't even see it.

What version of Photoshop are you running?

Your scratch disk is the free space on your hard drive that Photoshop uses when it doesn't have enough ram. Is your hard disk pretty full? If so, then you might want to think about doing one or both of the following:

First, you can back up some of your files onto external hard disks and clear some space on your internal hard drive. You will want to quit Photoshop and probably restart your machine after you do, then restart Photoshop and see if you can start working again.