Don't know bars from seedlings, but "I need seedling" does not seem to be valid English to me.
It certainly needs an article (or possessive adjective), e.g. "I need a seedling."
Report it. This is course is in beta, early beta at that. If something isn't right, a translation is missing, etc, report it.
I need a bar should be accepted. (I have reported this) E.g "I need a bar of soap" = Ninahitaji mche wa sabuni
"I need a seedling" is accepted now. But "I need the seedling" is rejected. Of course, it should also be accepted as it is a perfectly valid and frequently used statement in the context of gardening.