Hi Stefano,
It looks like you are trying to compute the inverse of a multivariate normal distribution via vpasolve. I will leave to the MATLAB Answers community to comment whether the mathematical setup in your code is correct. However, from the MATLAB side, I can tell you that the error message you are getting:
Error using bsxfun Operands must be numeric arrays.
Error in mvncdf (line 137) XL0 = bsxfun(@minus,XL,mu);
Is due to the fact that you are defining the variable "a" as a symbol, and passing in that symbol into the argument of "mvncdf" function to compute the inverse of the distribution. Unfortunately, "mvncdf" does not accept symbolic inputs because in one of the underlying function calls, it invokes "bsxfun" which errors out when one of the input arguments (in your case it is "XL" on line 137 of "mvncdf") is not numeric (i.e. here "XL" is a symbol--you may verify by placing a breakpoint in the debugger).
I am not sure if "vpasolve" is the right tool to solve for the inverse of the multi-variate normal CDF, as this CDF is not an algebraic function. In MATLAB we have the function " norminv " that can be used to find the inverse of the single-variable normal CDF. However, a multi-variate version does not exist yet.
