Hello Tomev18,
We understand you're experiencing an issue installing macOS Big Sur, and we'd like to get you pointed in the right direction for a resolution. Get started with the guidance listed here: How to use safe mode on your Mac
If the issue continues in safe mode we recommend relying on macOS Recovery. Those details can be seen here: How to reinstall macOS
You should not need to use a password to enter macOS Recovery unless your storage device is locked. You can learn more here: Encrypt and protect a storage device with a password in Disk Utility on Mac
You can select your current macOS version in the top left.
Take care.