Text highlighting not working in Word

Your keyboard can select text and the mouse works in Excel, so the hardware is fine; the fault is isolated to Word. Because Online Repair helps only briefly, an add-in, template, or customization may be reloading. Close Word, press Windows+R, run winword /safe, and test documents. If selection works, open File > Options > Add-ins, manage COM Add-ins, disable all of them, restart Word, then re-enable one at a time until the problem returns. If Safe Mode still fails, close Word and rename Normal.dotm in the user Templates folder so Word creates a template. Install Office and Windows updates, and test in another Windows profile. Do not keep repeating Online Repair without isolating the trigger. If the issue persists in Safe Mode and another profile, collect the Office build and application events, then reopen Microsoft support; that evidence points to an Office regression rather than an add-in.