Checkout is a noun and an adjective. The corresponding verb is two words—check out. For example, when you are ready to check out at the grocery store, you wait in the checkout line. Or when you want to check out of your hotel after the standard time, you might ask for a late checkout.
The one-word form is sometimes hyphenated—check-out. But the unhyphenated form is more common in all the main varieties of English.
Checkout is a noun and an adjective—for example:
It’s the one about the supermarket checkout staffer who chides an elderly customer for not having recyclable bags. [Guardian]
The churchgoers won’t be seeing me, though, as I figure there’s just enough time for one more bath before checkout. [Sydney Morning Herald]
At the checkout, an apple-faced female cashier in her teens with the name tag “Chuck :)” notices us and our photographer. [National Post]
And check out is the verb—for example:
You can check out any time you like, but you can never leave the reach of copyright law. [Los Angeles Times]
If you’re looking for less extravagance, check out the Odeon cinemas. [Independent Online]
Also, check out 8mm which turns your phone into a superbly retro super8 camera. [Irish Times]