Your SAS code has several syntax errors. "^" should be "**", "b=10^6" should be "b=10E6", and the last "end;" generates an error as it is not needed. Those errors seem a bit more than cut-and-paste issues. Does the code really run in SAS?

I am no expert. It may have something to do with the fact that your model is not differentiable. Sorry I can't help you any further. Here is a code that gives an output, even though with a warning:

data = {{0.1, 1}, {0.5, 0.99245283}, {1, 0.981132075}, {2, 
    0.957629013}, {3, 0.94647978}, {5, 0.892872862}, {8, 
    0.825362586}, {10, 0.794884307}, {15, 0.737820069}, {30, 
    0.688456089}, {50, 0.710852149}, {100, 0.686350949}, {300, 
    0.630188679}, {500, 0.622243133}, {1500, 0.61197978}};
model = Piecewise[{{1, x < a}, {0, 
     x > b}, {(x^(d - 3) - b^(d - 3))/(a^(d - 3) - b^(d - 3)), True}}];
nlm = NonlinearModelFit[data, model, {{a, 1}, {b, 10^6}, {d, 2.8}}, x]

So. The case is that S is the relative saturation, b is the moment in which the saturation is 0 and a is the moment in which the saturation is 1, so that if x(that is the potential in a specific water content)>b saturation is equal to 0, and if the potential in a specific water content (x) is minor than a, so that the saturation is 1.

I think your code has work succesfully, I only changed the way you wrote the conditionals.

The following gives a result with a warning:

Clear[x, a, b, d];
data = {{0.1, 1}, {0.5, 0.99245283}, {1, 0.981132075}, {2, 
    0.957629013}, {3, 0.94647978}, {5, 0.892872862}, {8, 
    0.825362586}, {10, 0.794884307}, {15, 0.737820069}, {30, 
    0.688456089}, {50, 0.710852149}, {100, 0.686350949}, {300, 
    0.630188679}, {500, 0.622243133}, {1500, 0.61197978}};
model = Piecewise[{{0, x > a}, {1, 
     x < b}, {(x^(d - 3) - b^(d - 3))/(a^(d - 3) - b^(d - 3)), True}}];
nlm = NonlinearModelFit[data, model, {{a, 1}, {b, 10^6}, {d, 2.8}}, x]