I am using Windows 11 and cannot delete highlights that a few days old. It is possible to delete them a few minutes after there are done, but after one or two days, it is impossible.
Did you use Store Annotations in File? If so, that transfers them to the PDF file, and then they can't be edited in Zotero, as the warning says when you use that option. (That option is being removed in the next version.) You'd see a lock icon in the annotation popups if you've done this.
If you're viewing the PDF, it's on your computer. Right-click on the item and click Show File, and open it in an external reader. Or use File → "Import Annotations…" from the Zotero PDF reader to import the annotations back in.
If you're talking about iOS, tap the highlight, tap the "…" menu, and tap Delete Annotation.