"The personnel is fine."
In English personnel can be used as plural or singular. Personally I prefer the plural, but Duolingo seems to use singular by default.
Personnel can refer to "workers" (plural) or "group of workers" (singular group).
"The workers ARE fine", but
"The group of workers IS fine"
Yes, I suppose it's a moot point... Saying "the personnel is" just sounds a bit wrong to me.
You have to add the word "group" to use "workers" as a singular; you couldn't say "the workers is fine". I'd probably do something similar when using the word personnel, or use the word ARE instead of IS. Other people may differ :)