Driving Theory Test 4 in 1 Kit app accounts are not created automatically when you purchase the app, but are free to create within the app.
What is now called an "account" used only to be created when you joined the Pass Guarantee scheme for car or motorcycle theory tests - if you've used the app before but didn't join the Pass Guarantee scheme you might not have an account.
If you've chosen to login as an existing user and selected to "Continue with email" but don't then receive the verification email (even after checking your spam/junk folder) you most likely do not have an app account.
Either tap the "Create new account" button, or restart the Driving Theory Test 4 in 1 Kit app then, when you see the app's "Let's get started" screen, enter your first and last names and tap the "→" button in the bottom right corner.