I quickly Googled relevant segments from your log:
main.INFO: Broken reference: the ‘global_notices’ element cannot be added as child to ‘global.notices’, because the latter doesn’t exist
main.CRITICAL: The product that was requested doesn’t exist. Verify the product and try again. {“exception”:"[object] (Magento\Framework\Exception\NoSuchEntityException(code: 0): The product that was requested doesn’t exist.
Verify the product and try again. at /var/www/html/vendor/magento/module-catalog/Model/ProductRepository.php:290
I found a similar error on Google to the one in your system.log. It suggests that you reset your file permissions. Basically, it suggests that you trying going through this post: How to diagnose and fix Ownerships and Permission issues
There was also a suggestion to check your PHP log. Check the timestamps to the time of the error as you did before. The location of this file requires root to access it and varies based on your setup. Mine is found in /var/log/apache2/error.log