I have already read a lot of topic about this issue, but nothing has worked.

I have my database with some variable. I want to calculate the marginal effect of W for a person with some characteristics.
This is my code :
margins, at(a=1981 b=1 c=1 d=1957 e=13 f=3 X=2)
But when I do that, Stata displays a message : variable X not found in list of covariates. What can I do ?