Validation Error Handling - DRY Pattern Implementation
public static class ExistsHelper
{
    public static bool Country(int id)
    {
        return (id &gt; 10) ? true : false;
    }

    public static bool State(int id)
    {
        return (id &gt; 10) ? true : false;
    }

    public static bool City(int id)
    {
        return (id &gt; 10) ? true : false;
    }
}  


The usage of a ternary here is senseless, you can just return the evaluated condition like so
public static class ExistsHelper
{
    public static bool Country(int id)
    {
        return (id &gt; 10);
    }

    public static bool State(int id)
    {
        return (id &gt; 10);
    }

    public static bool City(int id)
    {
        return (id &gt; 10);
    }
}  

but looking at it, it seems it doesn't matter what &quot;type&quot;  you are validating because they all have the same condition. You could simply use only one method and extract the magic number 10 into a meaningful constant.

ValidateNotExistImproved
Here you are using a ternary as well which isn't needed because a simple if statement would be enough and more readable like so
public static string Validate(EntityTypeEnum type, int id)
{
    switch (type)
    {
        case EntityTypeEnum.Country:
            if (!ExistsHelper.Country(id))
            {
                return NotExistErrorCodeConfig.Country;
            }
            break;

        case EntityTypeEnum.State:
            if (!ExistsHelper.State(id))
            {
                return NotExistErrorCodeConfig.State;
            }
            break;

        case EntityTypeEnum.City:
            if (!ExistsHelper.City(id))
            {
                return NotExistErrorCodeConfig.City ;
            }
            break;
    }

    return string.Empty;
}