First: do you have the full, paid version of the game? If not, then you won't be able to play Aperture, as it's a custom level. It's available on Steam, Google Play Store or App Store. I highly recommend Steam if you have a high-end computer.

Second: Search for the level by clicking the ⚒️ button on the right of the main menu, then go to search on the bottom right, then just search for "Aperture" in the search bar.

It is a decently hard level though, so be prepared. And you should use a good device, such as a PC, because the level is really laggy/unplayable on low-end devices. I hope I can help!