You have not wired a constant to the N count terminal of a For Loop. If this terminal is not wired, LabVIEW does not know how many times to execute the For Loop.
To correct this error, right-click the N count terminal and select Create»Constant from the shortcut menu. Enter the number of times you want the For Loop to execute.
Alternatively, you can wire an indexing input, such as an array, to the For Loop. The For Loop executes once for every element in the array.