I'm signed in on the website but the app still asks me to sign in

That is expected. Browser sessions are shared between Creta websites, but an installed app keeps its own sign-in on that device — signing in once inside the app fixes it for good on that machine or phone.

Being asked again in the browser usually means one of these:

• A different browser, a different profile, or a private window — sessions do not cross those boundaries.
• Cookies were cleared, or the browser is set to clear them on exit.
• You changed your password, which deliberately signs out every other session.

If signing in appears to succeed but bounces you straight back to the sign-in form, clear cookies for the site once and sign in again — that clears an old session cookie left behind by an earlier visit.