Eraser tool missing from toolbox

Yep — if you mean the Eraser/Remove tool in the Mac’s Markup toolbox, the first thing to know is that macOS Sonoma 14 does not show the same tools for every file type.

Apple says the available Markup tools vary depending on what you have open. 

Mark up files on Mac

Try this:

1. Open the image in Preview, rather than editing it from a Quick Look window.
2. Click Show Markup Toolbar (the little toolbox/pen icon).
3. Look through the toolbar and its More (…) options.
4. If you're trying to erase part of an image, Sonoma's Preview may instead give you Selection, Lasso, Smart Lasso, or Instant Alpha tools. Apple's current Sonoma documentation lists those rather than a conventional freehand eraser. 

Annotate an image in Preview on Mac