Previous responses are correct. There is no Micro Adjustment feature on the T8i or any of the other Rebel (or "Kiss" or EOS xxxD, xxxxD) series cameras. That feature is only found on the EOS xxD and xD series DSLRs. (It is also not a feature or even necessary on any of the mirrorless cameras.)
Honestly, I would forget about it. Throw that focus calibration tool in a drawer, go shoot and enjoy your camera.
You still haven't told us what lens you were testing for the sample photo. Maybe the EF-S 18-55mm? Or perhaps the EF-S 18-135mm? Those and the EF-S 55-250mm simply aren't critical of precision focus, the way large aperture lenses are. At "normal" shooting distances with those lenses the natural depth of field of f/3.5 to f/5.6 lens apertures will obscure any minor focus error.
In fact, you probably should try stopping down a little if sharp images are your goal. Most lenses are sharpest a stop or so smaller than their max aperture. And that stopping down will further increase your depth of field, making focus precision even less crucial.
By all means test your lenses.... go shoot a highly detailed, flat surface like a weathered fence or brick wall... keep parallel to the subject... shoot at different focal lengths and apertures to "learn" where you're lenses perform best.
Other things that can help sharpness...
- Look at your camera holding technique and stance. Is it the best it can be?
- Read up on and learn to use "hyperfocal focusing distances". There are apps you can put on your phone to help calculate the ideal distance, but it's also pretty easy to learn to eyeball it.
- Remove any and all filters that aren't serving a purpose.
- Only use very high quality, multi-coated filters.
- Keep your sensor clean. Of course keep the lens clean, too (especially the rear element).
- If shooting very close, macro or nearly so, where depth of field is shallow, switch to Live View. This uses a different form of autofocus... slower, but more precise.
- Manual focus using Live View, using the magnification feature to check focus.
- Use a good quality tripod and don't over-extend it.
- Use a remote release (or use the self timer to delay shutter release, so you aren't touching the camera during exposure).
- Keep IS...image stabilization... turned on (most lenses... there are five older Canon lenses where it needs to be turned off when on a tripod).
- Learn "focus stacking" for macro and some landscape/architectural photography.
- Avoid using too small lens aperture, which can cause diffraction that softens images. With a 24MP, APS-C camera like yours, I would probably try to use no smaller than f/11.
- Don't evaluate image sharpness and focus accuracy at ridiculously high magnifications on your computer monitor. A 24MP image "at 100%" on a typical monitor is like making a 3 foot by 5 foot print, then viewing it from 18 or 20" away. If you looked at the Mona Lisa that close, all you would see is Rembrandt's brush strokes! It's fine to use 100, 200% or higher while doing fine, precise image edits... but back off to maybe 24% or 33% for a more realistic evaluation of images sharpness and focus
- Sharpen your images in post-processing. (This should be one of your last steps... after the image has been sized for its intended use.)
