If you receive a notification to your email that an invitation has bounced back it is because the email address you invited is either incorrect (e.g. typo) or is rejecting email (e.g. it has some blocking software).
In the first case, you can delete the invitation. You can find it by searching for it, or you can click on the number in the Pending invitations column
Click the red Delete button and it will add the course licence back to the spare course licences total. Now you can invite them using the correct email.
In the second case, if their email address is blocking, you can either:
1) ask them to whitelist emails from @drivingtests.co.nz, in which case click the Resend invitation button afterwards.
2) ask them for a different email address, delete the invitation and reinvite them using the new email.