The sentence "it won't affect" is correct and usable in written English. You can use it when you want to reassure someone that something will not have any effect or impact. For example: "Don't worry, changing the date of the meeting won't affect our plans at all.".
It won't affect anything".
It won't affect your score.
It won't affect our borrowing costs.
"It won't affect his kids," she added.
"It won't affect Russian business in general.
Hopefully it won't affect my walk.
But it won't affect the script".
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com