Merchant accounts are protected with multi-factor authentication. As well as your password, you confirm each sign-in with a second step, such as a one-time code.
You will be asked to set this up the first time you sign in. If you are given backup codes, keep them somewhere safe.
If you lose access to your device
Ask another administrator on your merchant account to reset your multi-factor setup. If you are the only administrator, contact support. We will verify your identity before restoring access, which can take a little time. This protects your account.