Let me guess, what your question might mean: The message
is shown, when you try to run this line:
Then the message is clear: The variable "U" is not existing yet. The posted piece of code does not clarify where this variable might be coming from.
Speculation: Perhaps you have defined U in the inputs, but started the function using the "Start" button (the green triangle). This cannot work, because the input must be provided to define U.
Best Answer