Wrong email or password
Double-check that you're using the email address your invite was sent to. If you're sure, use "Forgot password" to reset.
SSO email mismatch
If your realm uses SSO, your Carerealm email must exactly match the Google / Microsoft / Apple address you're signing in with. A different email — even with the same domain — won't link.
2FA code rejected
If your authenticator app's codes are rejected, the most common cause is your device's clock being off. Time-based codes are very picky about clock accuracy. Sync your device time and try again. If that doesn't fix it, use one of your recovery codes (you saved them, right?).
Lost 2FA recovery codes
Ask a realm admin to reset your 2FA. They can disable it on your account so you can sign in normally and re-enable it fresh.
Invite link expired
Invitations expire after 30 days. Ask your manager to resend.