我在采用NLP过程进行似然函数求解时，出现以下问题：
NOTE: Your code contains 7 program statements.
NOTE: Gradient is computed using analytic formulas.
NOTE: Hessian is computed using analytic formulas.
ERROR: Execution Errors for _OBS_= 2 :
ERROR: There are references to missing variables when the program code is executed for _OBS_= 2
WARNING: Your program statements cannot be executed completely.
WARNING: In a total of 1 calls an error occurred during execution of the program statements. NLP attempted to recover by using
         a shorter step size.

我的编程如下所示：
而我在用蒙特卡洛模拟做的时候，编程如下：
用MCMC过程能够得到结果，但是为何NLP就得不到结果了呢？