Your VI seems inside out. Usually the while loop is on the outside, and the case structure is on the inside. Right now the only case that can ever execute is the one that is valid right when the code starts to run.
Your comparisons with zero don't make any sense. You have an = "0" , < "0", and a > "0". You are treating the zero like it is a value, but it is not. It is a string. Less than and greater than comparisons with strings are like dealing with alphabetizing words. They have nothing to do with the value of the number that the string is representing.
When you say something like "I keep getting an error", you need to provide some detail as to what that error is. Otherwise we can only guess which one thing of millions of possible errors it might be.
