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Its time to eliminate the easy stuff first. New battery perhaps even the charger. Make sure they are real Canon batteries. Fully charged. A brand new SD card (no micro SD's) of known good quality exactly the one recommend in your manual. Reset the T6i to factory settings. Menus, tools, clear all settings and all custom settings, too.
Now give it a try. Works, great, Still doesn't work, call Canon.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
04/20/2023: New firmware version 1.4.1 is available for EOS R3
03/30/2023: New firmware updates are available.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM