After Calamares finishes and the machine reboots, a first-boot setup wizard runs (language, account, timezone, look) and finally hands off to a root script that actually creates your account and applies your choices. If that last step fails, you're dropped with an error instead of a silent freeze.
Check the log:
~/Desktop/post-install.log(on thedefaultuser's desktop, since your real account doesn't exist yet at that point)/tmp/post-install-error- just the one-line reason it stopped, if it did
If the wizard's own finish page shows a QR code, that's a link to the crash log for pasting/sharing when you report it - it only appears when something failed and you're online.
Common causes:
- No internet during setup - some steps (clock sync, downloading extra components) are skipped rather than failing, but if you expected them to run, connect to WiFi/ethernet first and redo setup.
- An invalid or reserved username picked on the account step.
If you're stuck at a black screen with no error and no QR code, switch to a TTY (Ctrl+Alt+F3) and check ~/Desktop/post-install.log from there instead.