In this simple food chain, the organism that converts solar energy into usable chemical energy is missing. that would be the?

The correct answer is Producers.

Producers occupy the first trophic level in a food chain that begins the process of energy flow in an ecosystem.

They can also be referred as autotrophs as they synthesise their own food using energy from sunlight (photosynthesis).

They convert solar energy into chemical energy (ATP and NADPH) that is finally utilized in the formation of food (carbohydrate).

Example of producers- Green plants and algae.