Why a slug can be unavailable even when you cannot see it in your account.
Slugs are unique per domain. Someone else on the shared domain may already hold it — moving the link to your own custom domain makes the whole namespace yours.
The slug may be reserved. Anything that would collide with a platform route is kept back so a link cannot shadow a real page.
You may already own it on an archived or disabled link. Archived links still hold their slug.
Changing the capitalisation does not free it up. Availability is checked without regard to case, so Launch is taken the moment launch is.
A correct password sets a cookie scoped to that link. If the browser refuses cookies — strict privacy mode, an extension, or an in-app webview — the redirect immediately asks again because there is nothing to check.
Open the link in a normal browser window rather than an app’s built-in browser.
Allow cookies for the short domain.
Repeated wrong answers are rate limited; wait for the window named in the message rather than retrying.