error C2280: attempting to reference a deleted function WHILE DECLARING A C++ STRUCT

I have the following C++ code (in VS2017),

struct OptionA
{
    std::string s1;
};

struct OptionB
{
    std::string s2;
};

struct Inner
{
    int b;
    union {
        OptionA optA;
        OptionB optB;
    }Options;

};

When I tried to declare this struct:

void function()
{
   Outer obj; 
}

I get the compilation error:

error C2280: 'Outer::Outer(void)': attempting to reference a deleted function

This way of declaring a struct should work fine. I feel this error is something to do with the constructor of the structure. How do I solve this issue ?