Both U32 and S32 indicate that your NumPy array is a string array, not a number array. Check whether there are strings in the dataset. If there are, just delete them. In NumPy array, as long as one item is a string, the type returned by the array is a string array.
If you need to convert numpy to floating-point number, please refer to the code:
train= train.astype (float)
train_ target = train_ target.astype (float)