Usually this error has to do with what you have wired to the "channel" input parameter of AI Config.
If you have created a virtual channel, make sure that you have spelled it correctly with the correct case sensitivity.
Note that the channel input is an array of strings or an array of the DAQ Channel Name control (LabVIEW 6 and later). Make sure that the other elements in the array are correct, as well. The left side of the array object has an increment button with a number. That number corresponds to the index of the array where 0 is the first element in your array. It is a common mistake that a user will have the correct channel string entered in element 0 of the array, but an incorrect channel string in another element of the array that is not being displayed by default. Use the increment buttons to scroll through several other elements of the array to ensure that they are also correct, or uninitialized (grayed out).
Moreover, with the string control, it is possible that you hit the key which may appear that the data you typed previously disappeared. However, the data is still there, it is just hidden from the view (depending on the size of the displayed characters). The key acted as the end-of-line key to bring you to another line of the string control. Click in the string control and use the arrow or delete key to make sure that there is no residual data in that string array element.
