You are in the middle of a brake job and the new pad simply will not go in - too tight, won't seat, or won't clear the rotor. It is a common mid-job frustration with a short list of causes. Here is how to sort it out.
The piston is not compressed
The most common reason: the caliper piston is still extended from the old worn pads, so there is no room for the thicker new pad. You must compress the caliper piston fully back into its bore before the new pad and rotor will fit. On many rear calipers the piston must be turned in, not just pushed - a special tool is needed.
Rust or debris in the bracket
Corrosion and old crud built up in the caliper bracket where the pad slides can leave no room for the new pad's ears - see bracket buildup. Clean the bracket abutment surfaces down to bare metal and fit new abutment clips - the pad should then slide in freely; see when to replace hardware.
Wrong pad for the vehicle
If the pad shape or thickness is simply wrong, it will not seat - brake pads are not universal; see are brake pads universal. Double-check the part number matches your exact year, make, model, and brake option. A pad for a different caliper will never fit right.
Work through it in order
Check the part is correct, compress (or wind in) the piston fully, and clean and re-clip the bracket. With those three done, a correct pad seats easily. Finish with quality pads, fresh hardware, and proper bedding.