。
I want to check out at 6 tomorrow morning.
Literal
Tomorrow-morning 6-o'clock [at-に] checkout [object-を] do want.
Hotel phrase. したい is the auxiliary ~たい attached to する — 'want to do.' A softer alternative would add のですが (チェックアウトしたいのですが); without it the request is slightly more direct, which is fine in customer-facing contexts but less typical of polished service language.