Ah damn! Just 2 requirements.

Radish is winter, and we're in summer. This seed only produces 1 crop, meaning you needed to have planted 9 seeds to complete the quest.
From what I see, you have 4 radish seeds.
If you chose to leave this Quest until the coming Winter, then you must buy a radish seed as you're short of 1 more. Meaning, you'll plant 5 and harvest 5, then this quest will be deemed complete.

I'm sorry, hope you understand what I'm saying? Please Ask More