warning C4018: '<' : signed/unsigned mismatch?

As the warning says, this is because of the comparison because of the signed (i.e. m_nDarksideEffectAttCnt, iIndex) and unsigned(i.e m_DuelWatchUserList.size()) types.

In c++20 exactly for this, we have std::cmp_less

#include <utility> // std::cmp_less

if (std::cmp_less(m_nDarksideEffectAttCnt, vec.size()))
{
    // 
}

This also covers the case, if we mistakenly static_cast the -1 (i.e. int)to unsigned int. That is, the following will not give you an error:

static_assert(1u < -1);

But the following will

static_assert(std::cmp_less(1u, -1)); // error

Also check out the other functions

std::cmp_equal
std::cmp_not_equal
std::cmp_greater
std::cmp_less_equal
std::cmp_greater_equal

m_nDarksideEffectAttCnt and iIndex are of signed types, while (the standard library) size() functions returns an unsigned type (std::size_t).

You can get rid of the warning by changing types of m_nDarksideEffectAttCnt and iIndex to std::size_t or by casting in the comparisons:

if(static_cast<std::size_t>(m_nDarksideEffectAttCnt) < m_DarkTargetIndex.size())
//...
if (m_DuelWatchUserList.size() <= static_cast<std::size_t>(iIndex))

or if iIndex and m_nDarksideEffectAttCnt can have negative values:

if(m_nDarksideEffectAttCnt < static_cast<long long>(m_DarkTargetIndex.size())
//...
if (static_cast<long long>(m_DuelWatchUserList.size()) <= iIndex)