Looks like it all stem from this
-(10/20-1)^0.88
or more simply
-(-0.5)^0.88
R raises -0.5 to the power 0.88 which is an imaginary number.
For some reason, Excel applies the negative to the quantity before the exponent, so is evaluating 0.5^0.88.
I'd take a look at that part of the equation and re-write so it is more explicit what that first "-" is supposed to do.