Using existential quantifier within implication

Question

I had used an existential quantifier within an implication, as below:

$$\exists ~ \sigma_{opt}~~~ \wedge~~~ n \in R^{+}~ ,~0<n<1 \rightarrow ~~ \sigma_{opt} = n$$

I've, recently, found that it is illegal to do that and just universal quantifier could be used in this situation. Could anybody explain the reason?