The bad periods are unavoidable, but they don't last forever. Much like good periods, they are easy come easy go. The hardest part about work is getting the job. Now that you have it, all you can do is to do your best. I don't know how literally you mean you cant say a word but unless you are extremely severe, we are rarely completely unable to get out a word. I don't know what all your duties include, but I can guess the main one is telling the customer their total. You can take your time and you can stutter on it. Most check out screens show the total now and customers know how/when to pay. I never worked as a cashier before, but I have had jobs where I had really bad periods of speech. There's not much to really "do" that wouldn't make it worse, so I bear the tough times when they come just like I embrace the easy time. It won't stay tough forever. You are allowed to take your time and stutter.