I'm really confused about this one, does it mean "Stay healthy"? But if it does mean that shouldn't it be に instead of で？
健康 is a na-adjective, and ～でいる is the correct way to say "to stay [state]". でいられる is its potential form.
- What is the difference between でいる and である in this example?
- What is the meaning/grammar behind noun + でいる?
- ～ないでいる verb ending
to be able to stay healthy
～にいる means "to be at/in [place]" (e.g., 日本にいる = "to be in Japan"), but 健康 is not a place.