Thank you for your response.

I have ZenCart 1.5.6c & I installed Image-Handler-5.1.8.

Below is the only file in log folder.

[12-Aug-2020 11:28:59 Australia/Sydney] PHP Fatal error: Uncaught Error: Class 'FualSlimboxObserver' not found in /home2/--------/public_html/--------/includes/autoload_func.php:79
Stack trace:
#0 /home2/--------/public_html/--------/includes/application_top.php(170): require()
#1 /home2/--------/public_html/--------/index.php(26): require('/home2/----------..')
#2 {main}
thrown in /home2/--------/public_html/----------/includes/autoload_func.php on line 79

[12-Aug-2020 11:28:59 Australia/Sydney] Request URI: /index.php?main_page=index&cPath=17, IP address: 114.119.100.100
--> PHP Fatal error: Uncaught Error: Class 'FualSlimboxObserver' not found in /home2/--------/public_html/--------/includes/autoload_func.php:79
Stack trace:
#0 /home2/--------/public_html/--------/includes/application_top.php(170): require()
#1 /home2/--------/public_html/--------/index.php(26): require('/home2/--------...')
#2 {main}
thrown in /home2/--------/public_html/--------/includes/autoload_func.php on line 79.