Good morning to everyone!
Is it possible to set a left margin for Notepad++?
Notepad++ is a text editor not a word processor so it has no concept of a left-margin. Note however you can set margins when printing (see Settings (menu) -> Preferences… -> Print (box on left), but I don’t think this is what you are asking about.
Yes it is possible to set the left margin of the whole window.
But you should try to describe the exact result you want to achieve,
or ideally make a screenshot. Otherwise it can result in misunderstanding.