If you are indeed on an older Android OS version that still requires an unlocking code, make sure your FP6 is connected to the internet (necessary for code verification). This might actually be the better approach given that an older Android OS version makes sure that the /e/OS version you install has a more recent Android security patch level – so you have no issues with Android’s rollback protection.
Thanks for your answers, actually my fairphone 6+ is running on Android 16, so this shoud be no problem. As described, i had wifi on and somehow it seems to have worked.
Unfortunately the rollback protection mechanism is not about (major) Android versions, but the monthly security patches. In theory, you could go back from Android 16 to Android 15 and still not encounter rollback protection as long as the target system has the same (or newer) Android Security Patches date. The newest /e/OS version 4.3 should arrive any other day this week, so if you can wait 2-3 days more, you should be super-safe.