Why can't we use Centrifugal Pumps for Viscous Fluids ?

Centrifugal pumps are used widely to pump liquids such as water.  They cannot be used to pump viscous liquids.
Impeller of a screw pump (positive displacement)

The high viscosity of the liquid will cause increased friction losses between the impeller and the liquid.  The friction losses result in reduced head and the pump efficiency reduces.  There is a sharp reduction in the pump flow as the viscosity of the medium increases.

Besides, the flow inside the pump will not be streamlined and this increases the losses and loading in the impeller.

Hence, the centrifugal pump cannot be used to pump viscous liquids.  Viscous fluids are pumped by positive displacement pumps.