Well OP, your question seems a bit defeatist, but of course you're answering it within the context of your own experiences and beliefs.
I am sure the number of people who are forever unemployable and unsuitable for volunteer work in any position in all circumstances is quite small.
The number of people who are employable in specific circumstances would be much higher. Many in this group could work if the circumstances were different (such if they could find a flexible employer and a position well-suited to their strengths and weaknesses, could create their own small business, or if they lived without any family support or were in a country without a strong social security system and had no other choice).
The question is, which of these groups are you in? If it's the second group, is there anything you can work towards to find something that fits for you?
Engaging in meaningful, productive activity doesn't need to be working full-time - it could be part-time or casual work, volunteering, self-employment, or study. When each of us do what we can, in the way we can, it's good for us and helps contribute to society.