If you carefully look at the documentation for Reduce you should see that it has

Reduce[expression,thingToSolveFor,domain]

In your Reduce above it thinks

0<= VL*Ishp+(1-ksag)*Vs* Sqrt[4*VL^2-3*VL^2*Cos[fai]^2+Ishp^2*ZL^2+ 2*Ishp*VL*ZL*Cos[fai]]/ZL

is your expression and

VL*Ishp+(1-ksag)*Vs* Sqrt[4*VL^2-3*VL^2*Cos[fai]^2+Ishp^2*ZL^2+2*Ishp*VL*ZL*Cos[fai]]/ZL<= VL^2/ZL*Cos[fai]

is the thing you want to solve for and

Ishp

is the domain.

Change your code to put your two expressions inside {} like this

Reduce[{0<= VL*Ishp+(1-ksag)*Vs* Sqrt[4*VL^2-3*VL^2*Cos[fai]^2+Ishp^2*ZL^2+ 2*Ishp*VL*ZL*Cos[fai]]/ZL ,
  VL*Ishp+(1-ksag)*Vs* Sqrt[4*VL^2-3*VL^2*Cos[fai]^2+Ishp^2*ZL^2+ 2*Ishp*VL*ZL*Cos[fai]]/ZL<= VL^2/ZL*Cos[fai]},
  Ishp] 

and you avoid this error.