。
Someone who can't do that is just plain unfit for the top spot.
Literal
That [obj-が] cannot-do person [topic-は] top [to-には] unsuitable-person [copula-なのである].
ができない uses が to mark the ability-object — the standard pattern with potential predicates. 不適任者 is a transparent compound 不 'not' + 適任 'suitable assignment' + 者 'person' = 'unsuitable person / wrong person for the job.' The formal copula ending なのである is the most elevated variant of the explanatory copula — なのだ's written counterpart, common in opinion pieces and essays. The sentence lands a judgment with authoritative finality.