# Primal solution exists but dual does not

I am working on the follwoing nonlinear model.

Min z=10(1-$$\exp$$(−3x)

)

subject to:

x $$\leq$$ 3

When I solve this problem on LINGO, I got the message "dual solution does not exist but primal solution will be shown". 1) Why there is no dual solution? How can I prove it ? How can I prove that there is no dual solution ?

2) How can we interpret all this case , I mean there is no solution for dual problem but there is only for primal solution.