Internal Speakers not showing in Output of Sound application As shown in the screenshot only the Headphones are selectable:



But no headset is plugged in.

I have plugged and unplugged the headset several times to see whether the Mac thinks it were plugged in and maybe to get un-confused: no difference.

While I could try to reboot it would be helpful to understand if there were some mechanism to reset macOS "understanding" of the headset in/out.
Explanation

If the sound output device is always displayed as 'Headphones' there are usually three explanations for it:


you have headphones plugged in
the tip of a broken headphone jack still sits in the Mac's audio jack
the Mac's audio jack is defective/broken




Possible solution

Please check with a flashlight whether you can see any debris in the audio jack. If debris/dust builds up it may trigger the electrical contact that makes your Mac switch from Internal Speakers to headphones.

Try to gently blow it out or pick it with a tweezer.