A Maximum Function For Various Type Arbitrary Nested Iterable Implementation in C++
Using the right concept
Indeed, Summable is the wrong concept to use if you want to check if the value type is such that you can calculate the maximum of two values. Have a look at the documentation of std::max(). In particular, it requires that the type is LessThanComparable. So you could create a concept to test that. There is already a concept std::totally_ordered, which is a bit more strict than necessary.
Simplify Max()
You do an elaborate dance to get the right type for maxValue, but why not just use auto? Also, use std::max() to calculate the maximum for you:
template&lt;class T&gt;  requires Iterable&lt;T&gt;
static inline auto Max(const T&amp; numbers)
{
    auto maxValue = Max(numbers.at(0));

    for (auto&amp; element : numbers)
    {
        maxValue = std::max(maxValue, Max(element));
    }

    return maxValue;
}