If you are having trouble sliding the rotor into the caliper when installing your wheel, this is generally caused by the lever being pulled while no rotor was in place. It can happen by mistake, but it is important to be careful and never do this! You will need to remove the pads and push the pistons back, then reinstall the pads and assemble as normal.

Here is a very straightforward & descriptive YouTube tutorial video on how to do that: 

To avoid this happening, when transporting or moving the bike with the wheel out, it's best to use a bleed block or pad spacer. Here is an example:

Pad Spacer