Mellow Mallow seed cannot be planted

Mellow Mallows are decorative plants - you should be planting them in flower pots, not farm tiles.

You are right, it's present in flower pot. The issue is just some tooltip text then, it can be seen, for example, when you choose some plant and check its requirements - it says "farm plot" is one of actual farms.

Not only does the tooltip say it should be planted in a farm plot, but it also says that it grows faster when domesticated.  That's not normally something decorative plants do.  The description also mentions that you can easily grow more from cuttings, but you can't.