A common issue when attempting to register your YubiKey with services that use Yubico OTP, such as LastPass, is that the OTP on your YubiKey is invalid. You can verify if you have a valid OTP at our demo website. If you receive the error Failed to validate OTP, please click the OTP itself (e.g. vvccccbeljhhedkthdvthktvkkfullnfvitkicdvveji) under Previously validated OTPs, to reveal a more specific error message (example below).
If this error message is "NO_VALID_ANSWERS", as shown in the above screenshot, then the credential used to generate the OTP is not known to our servers, meaning it will need to be uploaded (LastPass uses our servers for Yubico OTP validation). This can be done here, but if you do not have the parameters needed to fill out the form on this page, you can alternatively program and upload a new credential; see this guide for instructions.
If the revealed error message is something other than "NO_VALID_ANSWERS", please send it to us in a support ticket here.