The language code uk is already assigned to another language — specifically, to the default pre-configured Ukrainian language in WPML.
To make a change, activate Ukrainian in WPML → Languages. Edit the Ukrainian language and change the language name to English, the locale to English, and the rest to English.
You might get a warning message stating that the English locale is already in use when attempting to change it in Ukrainian language. In that case, you need to edit the English language and change the default locale to something else. Once the default locale is free, you can assign it to another language.
Since the language code uk already exists and cannot be changed, that is the only way to do it.