You can install Bitwarden’s Portable Desktop app on a USB, then launch the portable app while disconnected from the internet, and use the Password Generator tool there.
If you want a simper solution, there are other password/passphrase generators available that can be downloaded for off-line use (from sources other than Bitwarden). Here is one that I like:
https://passhelp.github.io/generator/
Just right-click in the page and choose Save As to save locally as an .html file, which you will then be able to open while off-line.
If you want a wider selection, check out Aaron Toponce’s “audited” password/passphrase generator list, and choose among those that have a value of “Client” in the “Generator” column (this means that the passwords are generated client-side):