It's not a problem if you have a step through bike or your bike doesn't have a traditional cross bar. You can still load your bike securely.
Most wheel support bike carriers use arms that attach to the frame of the bike. If you can’t attach the arm to the cross bar, it’s fine to attach it to another part of the bike, e.g. the seat post or handlebar stem. If you are carrying more than two bikes, it's usually easier to attach these arms to different parts of the frame on each bike anyway.
If you are using a 'hang-on' bike carrier (where the bike frame is held by two arms) you will need some form of cross bar in order to be able to load the bike securely. Artificial cross bars are available if your bike doesn't have a traditional cross bar. You can see our range of artificial cross bars on the 'bike carrier accessory' pages. Artificial cross bars attach between the seat post and handlebar stem, you do need to check that a particular cross bar fits the distance between your seat and handlebars. You can find this information on the pages for the artificial cross bars (usually on the 'description' tab).