In order to be able to create the Snapchat pixel you need a Snapchat ad account. You can use an existing one, otherwise create a new ad account in the Snapchat business manager.
Open the Snapchat business manager at business.snapchat.com
Open Pixels in the menu on the left and then click Create Pixel
Give it a descriptive name and click Create.
Select the ad account and copy the pixel ID.
Paste and save the pixel ID into the Pixel Manager.
Optionally, enable automated matching in the Snapchat pixel.