if you input YES into this code:
input "say YES or NO", x
The calculator will multiply Y by E by S and if that value is greater then 0 the Disp "SUCCES" will be triggered. However when you input NO it will try to multiply N by O. N or O must have been assigned a value before you can do anything with it, even multiplying so the variable N or O does not exist. Just try to get the values of N and O by typing them into the homescreen, should give you the same error.
Anyways you'll have to understand that a calculator doesn't think like us and doesn't see the difference between a number and a string. So if I wrote this down:
Then you can immediately see that I want "HI" assigned to A such that whenever I call A I get "HI" returned.
So this code below should return HI don't you think?
Wrong! The calculator has very specific
variables for different data types. "HI" is called a string, "ASDjk4542" is also a string but HI isn't. A string is enclosed with quotation marks (altough you can leave the last one off to save space but thats a different story). Now to get back to my point, A is a variable but A does not save strings, it is specifically intended to store numbers
to be more specific, real numbers.
The above code is valid
the code underneath isn't
Just to be short:
A,B,C,D till Z and θ are variables intented to store real numbers
str0-str9 are intended to store strings
[A]-[J] are intented to store matrices
L1-L6 are intented to store lists
so for each piece of data there is a data type you must use to help the calculator distinguish what is a number and what is text. This is a thing you must understand because there are nearly 0 programming languages that don't use data types. Simply because a computer can't just know like we humans can that "AVC" is a string and 100 isn't, to a computer it is all data.