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I really don't know unless the seeds are not viable. While these can be finicky when it comes to germination, you should still have gotten something. And since you've been gardening this long, I would think the soil and growing conditions for this plant are being met. Very unusual. I wish I could be of more help, but all I can think of would be the seeds or the soil. You may want to try a soil test to see what this turns up.

Harvesting of greens can vary between variety, but you can harvest most greens by taking individual leaves, cutting the plant at the soil or pulling the entire plant.

https://www.gardeningknowhow.com/edible/vegetables/greens/leafy-garden-greens.htm