Without knowing how your calculator is viewing the negative exponent, and without knowing how your calculator computes inverses (I think), then there is no way for us to know why the calculator is returning the error message you report. My guess would be that the calculator is assuming you mean the square of the inverse, and its algorithm involves dividing by the determinant, which here is zero. But that's only a guess.
Meanwhile, you still haven't said what you mean by the negative square of a matrix. Please define this. Thank you.
Eliz.