If you're having trouble sliding the rotor into the caliper when installing your wheel, this is generally cause 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 video on how to do that: 

To avoid this happening, When transporting or moving the bike with the wheel out, its best to use a bleed block or pad spacer. Such as this one:

shipping spacer