You have a numpy array of strings, not floats. This is what is meant by dtype('<U9') -- a little endian encoded unicode string with up to 9 characters.
try:
return sum(np.asarray(listOfEmb, dtype=float)) / float(len(listOfEmb))
However, you don't need numpy here at all. You can really just do:
return sum(float(embedding) for embedding in listOfEmb) / len(listOfEmb)
Or if you're really set on using numpy.
return np.asarray(listOfEmb, dtype=float).mean()