You can get repellants but they also don't tend to like the smell of citrus so you could keep citrus smelling things round there while she's too young to go out and while she is getting used to using a litter tray.
Can you put her litter tray near the back door, somewhere private for her but where she can easily find it. maybe something which smells familiar to her near it to help her find it while she's little.
Gentle but persistent training is usually best, if she craps outside the tray, pick it up with a litter scoop and put it in the litter tray and her up, stressing a gentle but firm "no" toher then pop her in the tray near it and if she doesn't start covering it over, give her a clue.
Keep doing this and she should get the message.
Make sure the tray is cleaned out very regularly as cats won't tend to go in a dirty tray and this would discourage her.