Looking up...
彼はまだ未熟だ。
He is still immature.
Lacking experience, skill, or development; not fully matured.
新人の彼はまだ未熟なので、先輩に教えてあげてください。
Since he's a rookie, he's still immature, so please guide him.
Often used with まだ or まだまだ to stress ongoing immaturity.