How come my USPS express tracking number doesnt have letters?

Hello, ordered a package earlier today, it shipped around 3pm and it is not 10pm EST. It still wont show updated tracking info. It is saying that error message saying it cant be found. Also my number is only well numbers. It doesn't have the two letters in front shown on the USPS website for express mail. Is this normal to not have the letters and have info not available??