In the off chance that you unintentionally registered a passkey that is stored on your Android phone (instead of registering the Yubikey as intended), you could try the Bitwarden login process again, but when you get the Windows Security “Sign in with your passkey” prompt, just click “Next” without changing the selection to “Security key” (i.e., leave the selection as “iPhone, iPad, or Android device”). The follow whatever prompts you are given, and see if this works.
The only other thing I can think of is that your Yubikey NFC model as the ability to store multiple “resident” FIDO2 credentials. So there is perhaps some trick required to get the Yubikey to present the correct FIDO2 key. These subtleties are unfortunately beyond my expertise, as I only have experience with the “simple” Yubikey Security Key series, which does not store any “resident” keys (only a single, hardware-bound key). Maybe somebody lurking in this thread can chime in, or perhaps Bitwarden’s customer support will be able to provide some advice.
Tried creating a new account in the app but it won’t let me because it already has an account with my email address . . . .
This is expected. Do you have another email account that you can use?
If you only have access to your one email account, and if you’re ready to give up on your original account (i.e., you don’t want to wait to see if you get any solution from customer support tomorrow), then you can delete your original account to make it possible to set up a new one. To delete the account, just submit the account’s email address on the deletion form available at https://vault.bitwarden.com/#/recover-delete, then look for the confirmation email and follow the instructions to verify that you want to delete the account.