I copied this example from a website but it does not work.
<form oninput="result.value=parseInt(a.value)+parseInt(b.value)">
0<input type="range" name="b" value="50" />100 +<input type="number" name="a" value="10" /> =
<output name="result"></output>
</form>
Most likely, your browser is outdated and is not accepting <output> .
I recommend that you check your browser version to make sure it supports this tag. In this link you can see the versions where <output> is already supported and details: Browser Support
Another possibility is that your server is not accepting the tag; in some cases the tag is not working locally, this is due to different versions.
I recommend testing on other browsers and different versions, as the browser is more likely to be incompatible.
I hope I have helped.