I want download bitwarden for my old macbook.
MacOs HighSierra 10.13.6
According to archived versions of the Bitwarden Downloads page from the Wayback Machine, pre-Yosemite versions of macOS were supported only up to April 5, 2021. Thus you could try downloading and testing releases starting with version 1.49.1 and then working your way backwards.
However, there have been significant changes to the server-side software since 2021, so these old apps are unlikely to be compatible with the server version running on Bitwarden’s cloud computers. If you are self-hosting, you could try to download and install a contemporary server version.
You should also read some of the notices in the documentation:
Good luck!
The last version that works on MacOS 10.13.6 is “Version 2023.12.1 (16254)”; it is compatible with the current version of the VaultWarden self-hosted server, but I can’t speak to whether it works with BitWarden’s cloud service. I just went through the downloads from most recent backwards till I got one that would run on my old laptop with 10.13.6 on it. 
You can download it at https://github.com/bitwarden/clients/releases/tag/desktop-v2023.12.1.
Thank you very much