Some videos in the Training Portal show a padlock. There are two reasons.
The earlier videos aren't done yet
Training is watched in order, because later procedures build on earlier ones. Complete the videos above it, including acknowledging each document, and it unlocks.
The site's plan isn't published yet
Each video pairs with a procedure document for your site. If your Premises Controller hasn't finished and published that plan, the video stays locked because there's nothing for you to acknowledge yet. Clicking the locked video tells you which plan it's waiting on.
What to do
If you've completed everything above it and it's still locked, let your Premises Controller know which video it is. They can check the plan's status on the site dashboard.