If you are already a member, ensure this is an exam within your membership package.
Go to Main Menu - Account - Profile and check your membership level.
If it is within your membership level, please restart your device and or clear cache, plus see that all your browser and device updates are done (especially on apple).
If that fails try the steps on the FAQ page under the heading Login Issues before contacting us via the contact form on the website only.
Note: Often when a login expires your device will serve you cached copies of content, so you think you are logged in, when in fact you are not.
For almost all login issues a device restart fixes the problem and should be the first step taken.
Regards Matt & The PPE Crew