  Let $F/F_{0}$ be a quadratic extension of non-archimedean locally compact
fields of residue characteristic $p\neq 2$. Let $R$ be an algebraically closed
field of characteristic different from $p$. For $\pi$ a supercuspidal
representation of $G=\mathrm{GL}_{n}(F)$ over $R$ and $G^{\tau}$ a unitary
group in $n$ variables contained in $G$, we prove that $\pi$ is distinguished
by $G^{\tau}$ if and only if $\pi$ is Galois invariant. When $R=\mathbb{C}$ and
$F$ is a $p$-adic field, this result first as a conjecture proposed by Jacquet
was proved in 2010's by Feigon-Lapid-Offen by using global method. Our proof is
local which works for both complex case and $l$-modular case with $l\neq p$. We
further study the dimension of $\mathrm{Hom}_{G^{\tau}}(\pi,1)$ and show that
it is at most one.
