My brake pads are closed

If you cannot place the rotor inside the brake pads during assembly, you may have put pressure on the brake lever without your rotor or pad spacer installed. 

The brakes on Ariel Rider bikes are self-adjusting, so if you compress the pistons, they don't retract fully, which compensates for pad wear. 

When the pistons are compressed without a spacer or rotor between them, this can cause the pads to contact each other, preventing the installation of the wheel. 

This is easily corrected by inserting a flat tool between the pads and then prying the pads open. 

Please refer to this video for reference.