Why can't I ice cupcakes with a star nozzle :(

I don't know of anybody who uses royal icing on a cupcake.  You need to make up some buttercream and any fondant decorations go into that.

 

Any icing intended for piping MUST hold a soft peak when you dip a knife into it.  You can adjust the consistency by adding the liquid named in the recipe slowly and testing between additions.

 

In any case you have added too much water to that mix.  It should specify water in mL rather than tablespoons. What you have right now is almost a flooding consistency--you need to add more mix from a second package to rescue it.

I agree with BakingIrene (as always!) about the consistency of the icing and the fact that nobody (on our side of the Pacific) uses royal icing on cupcakes, although there is a lovely boiled meringue icing (my mom called it 7-minute icing) that would work nicely. Buttercream or cream cheese frosting works best and it needs to be a little bit thick - able to hold a peak, as BakingIrene says - for it to work on a cupcake. Just one other thing: when making royal icing, either from scratch or from a mix like you're doing, the tiniest speck of grease on any part of your equipment will keep the icing from setting up, no matter how little liquid you use. Hope you can find something that you'll like!

I agree with the others. You need to make butter cream icing to make swirls on cupcakes. Most people Use the 1M tip or the 2D tip for cupcakes. Good luck/Mary

Hey I'm from aus too, use buttercream
I use the Wilton recipe and works a treat everytime.
113g softened butter
113g fry master veggie shortening(only Coles sell it)
4 cups sifted icing sugar
Vanilla or whatever flavoring
2 to 3 tbs milk
Cream butter and shortening
Add half the icing sugar beat again
Add milk and flavoring beat again
Add remaining icing sugar beat until light and fluffy
So easy and works a treat and it will hold up using a large star nozzle as this is my new best friend and use it all the time.

That stuff is called "crusting buttercream" here on Cake Central.  Made with vegetable shortening, powdered sugar and water...many many threads worth of discussion.

It air dries with a crust.