In fact the rotating region diameter is exactly as same as the blades' tip diameter (196.77mm), as shown below:
This causes some difficulty for the meshing solver (something like zero thickness, but this is not a proper term here!). Anyway, I just slightly increased the diameter of the rotating region and problem solved!
Siavash is correct, when the rotating area intersects with your rotor it can cause meshing problems.
Choosing the diameter of the rotating area can be tricky, because it will set the size of your rotation "core". If you set it too large you might "erase" blade tip effects and get a wrong answer. But as you can see setting it too small is not a great choice either because you can have meshing errors.
Since this design appears to be something you drew up just for testing the capabilities of the software, I would also recommend you use this opportunity to play around with the size of the rotation region and see how it affects the flow.
Thanx so much both of you.!!
Seems to have done it!