azza225,
By placing waveform graph indicators at multiple points in your code I was able to find that an empty waveform was being passed into the 'Wavelet Denoise' Express VI on the first iteration of the loop, causing the error.
It seems to be due to the way you are manipulating arrays in your code. I was able to resolve the error by passing an initialized double numeric array of '0' into the loop (see picture), but you may want to revise your code later to improve the array manipulation. In general, it is not advisable to build arrays inside of while loops due to performance and stability errors that it may cause.
Patrick O.
Applications Engineering
National Instruments
