Hi!

Actually I don't get this error.  I get

--8<---------------cut here---------------start------------->8---
guix build: error: derivation `/gnu/store/78dk4v22xwzxrvl7wrjl1g67qrf1yd37-git-checkout.drv' has incorrect output `/gnu/store/7ygy97wz9d1zcbz3k2kg1ga9g389bd7b-git-checkout', should be `/gnu/store/mnkrx2xpxnqaizmylyya8ilvmym7w0b7-git-checkout'
--8<---------------cut here---------------end--------------->8---

This is because the URL cannot be empty.
Beyond that, you must specify a name and a version if I'm not mistaken.

-- 
Pierre Neidhardt
https://ambrevar.xyz/