Not every volunteer will be able to book on via the link. It may land in their junk, they may not use a computer, or it might just be slicker for you to book them onto training for them.
If this is the case, then from the training event page, assuming there's capacity, you should see the following box at the bottom of the training event page. It'll allow you to select a volunteer and add them to the listing without the need for the booking link.
The volunteer will still get the automatic email(s) with the joining instructions and a note will be added to their record. The CVT for the volunteer will also be told. You can select multiple volunteers at a time (please just don't select more than the capacity!)