I reset my master password and after that I'm now unable to login. I'm also unable to reset the password again.
When I try to login it says:
"Please review the information you entered and try again"
When I try to reset I'm presented with this on the confirm code screen, despite having a yubi-key attached to the account:
And after entering the confirm code and submitting the yubi-key code, it says the credential is invalid.
At this point I'm completely unable to access my account. Please help.
Do you remember the previous master password? If you just changed your master password within 30 days and remember the previous one, you can Revert the master password change by following the steps outlined here: https://lastpass.com/revert.php
If you do not remember the previous master password, you would want to try all of the possible recovery options detailed in this support article: https://support.logmeininc.com/lastpass/help/recover-your-lost-master-password-lp020010
The error message you are seeing means there isn't a recovery one-time password stored on that browser, and you would want to try using that recovery link in all possible browsers/devices to try account recovery via that method. Yubikeys are used only for multifactor authentication, not account recovery, so please be sure to try all of the possible recovery options in any and all browsers you've ever used to log in to LastPass.