I have looked all around with no avail. I opened blender today, and now am unable to delete any keyframes. When I hover over it in the timeline with the green line, then "delete keyframe" and confirm, it says "Successfully deleted 0 keyframes from camera." (for camera). How can I fix this? I have tried selecting on dope sheet, but for some reason nothing happens either. Thanks in advance!
Found the problem. Somehow all of the keyframes were locked in the Dope sheet, so I just unlocked them, and now I can delete them.
To unlock or lock a keyframe, you can do three ways.
1) click on the icon symbol to unlock the keyframes for the entire object
2) hover over the keyframes for the selected object press tab once, if grayed out the keys are locked if not then the keys are not locked.
3) if your mouse cursor is outside the timeline boarders (gray part of the timeline) with this icon greyed then those keyframes are locked.
If somehow you are still stuck, just go into the dope sheet then Shift + Right Click select the key(s) you want to delete
Next tap X while hovering over the dope sheet Select left click the "delete keyframes".
The timeline shows any and all keyframes. This includes keyframes that don't get removed when removing keyframes normally. Change one of you editor's window type (I suggest the timeline window, as it won't be needed) to the Dope Sheet Editor, and select any keyframes you want to remove, and press 'X' to delete them. I suggest you look further into the dope sheet editor, as it is very useful for advanced manipulation of keyframes like duplicating, moving, etc.