Error 20 is indicative of a mechanical malfunction that has caused a disturbance or lockup that could likely be caused by the mirror, shutter, or aperture assemblies.
Assuming you are using a Canon battery, if the procedure you outlined (which is what is recommended) does not resolve the issue, then your camera will need to be serviced.
If you are in the US, call 1-800-OK-CANON.
A Rebel SL2 should be current enough for service by Canon. Give the friendly folks at 1 (800) 652-2666 a call. They will instruct you how to get it evaluated and afterwards you can decide if it is worth it to you. If it should be out of service life there are third party service centers that can repair it. Midwest Camera is one a good one. Might be something simple you just won't know until they check it out.
You did do a full reset didn't you? You are using Canon brand batteries and charger? You have no off brand ("whole kit at Costco") accessories attached to the SL2? Off brand anythings can cause your error 20.
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.