Have you filled in your first name and last name in the Profile > Details page? Both must be filled in. You can use your company name in there if you want the emails to come from the company rather than you. The company logo will appear in emails.
Do you have a Fleet Driver Plan or another course? If you just purchased a Learner Driver Plus course, you only have one driver (yourself). Purchase at least two licences for any other course and it will upgrade you so that you can add other drivers.
If you already have a Fleet Driver Plan, have you got any spare drivers? If not, you can purchase some more or, if you are on a Guaranteed Drivers plan, delete an existing driver and this will give you a spare account. Remember to download that driver's data first, though, because once deleted, their information is gone forever. When you click on Invite Drivers, it should look something like this, with the highlighted figure being anything other than zero.
Are you the account holder? If you are a Manager or Driver, but not the Account Holder, you can't invite drivers.