Bad mistake on first few days

I was training on cashier for the first time and I was doing everything I was told to do. I gave change bills, 5 dollar bills AT MOST, i never gave change that was over 10 dollars. I thought i was doing just fine, my trainer said i was doing great too. At the end of my shift they counted the bills on my toll and i was 50 dollars missing. I DONT KNOW WHAT I DID WRONG. I made sure to mark bills over 20 and i was careful with giving change and counting the money. Is this a common mistake? I didn’t know what to say other than “Sorry i don’t know what happened i’m new” This got me stressing help