What's wrong with it? I bought it and wanted to plant, but received a system message that this kind of seeds doesn't exist!

That's been happening for awhile. :/

Quite a few of the seeds are glitched and won't let themselves be planted.

If you click on the Melix Seed in your inventory you will see an option to "Water Seed", click on it and it will instantly change into the Melix Flower. No garden planting necessary. (It's funny like that.)