  In this paper, we study the ideal approximation theory associated to almost
$n$-exact structures in the $n$-exangulated category. The notions of $n$-ideal
cotorsion pairs and $n$-$\mathbb{F}$-phantom morphisms are introduced and
studied. In particular, let $\mathscr{C}$ be an extriangulated category which
satisfies the condition (WIC) and $\mathcal{T}$ be a nicely embedded
$n$-cluster tilting subcategory of $\mathscr{C}$, we prove Salce's Lemma in
$\mathcal{T}$.
