Well, it can mean ’girl’ and then the difference is very overlapping, but flicka generally only refers to young girls whereas tjej can be used for older girls as well. However, you can also say tjej to mean girlfriend, but not flicka. If you use flicka then it sounds like you’re saying ’daughter’.
So, it could be translated to "gal" perhaps, when using it for older girls?
The point is that it's not a very good translation, and accepting it would definitely give a lot of learners the wrong idea - especially the many who are not native English speakers. The course isn't supposed to accept all technically correct translations, just those that are reasonably feasible. I really do not think "my girl" fits that criterion.