问题：
f1 by x1 x2 ; !x variable are categorical
f2 by u1 u2 ; !u variable are continuous
f1 with f2;
x1 with u2; !correlated residuals（两个外生观测变量的残差设置相关）
运行后提示错误：
ERROR in MODEL command:
Covariances for categorical, censored, count or nominal variables with other observed variables are not allowed.
任务：
能够正常运行！ 必须用mplus软件运行，AMOS或者lisrel操作很简单，我自己会。
悬赏链接：https://bbs.pinggu.org/thread-6621881-1-1.html