The two, by far, most common reasons for that error to show are that 1) you are not using the same email address as you used at initial registration, or 2) you are attempting to unlock a similar product but not the exact version/offer that your activation is for, see more detail below. You can also watch this one min. video about the registration process.
1) Because the system matches the activation code to the email you provide, it treats the combination of email and activation code as a logon. This means that each time you enter the activation code, you need to also the very same email address you used at initial registration.
2) If you for example have the A+ Certification Bundle (801 & 802) make sure to access only that A+ Bundle product (third in the menu list). If you try either of the individual A+ tests 220-801 or 220-802, then your Bundle code will throw an invalid code error. As far as the system is concerned there are three A+ products: A+ 220-801, A+ 220-802 and the Bundle. Eacah of these get a different activation code.