The most likely cause is the hot shoe’s flash-detect microswitch being stuck or blocked. On cameras like the 60D, that switch tells the camera an external flash is mounted; if it sticks, the camera disables the built-in flash and shows the “external flash is attached” message.

First, inspect the hot shoe carefully for grit or debris under the rails or around the switch area. If you can see dirt, try removing it very gently with a non-metal tool such as a wooden toothpick/cocktail stick. Be careful not to bend contacts or force anything.

If nothing obvious is visible, or cleaning doesn’t help, the safest solution is to have the camera checked by an authorized Canon repair center.

Avoid forceful workarounds like prying the flash up or hitting it; they may seem to work temporarily but can damage the flash mechanism or hot shoe.