There are 2 probable solutions.
1. The images are "protected" by the camera & will show "locked" when you try to erase them.
2. The SD card in the camera is locked.
The solution to 1 is to unlock the images in the playback menu. See page 45 of the manual.
Solution 2 is to remove the card from the camera & check the "lock" slider that you can see on the side (although no all cards have this). Set the lock off.
You can download the manual from the first link on the following page: