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Thank you Steve, the only option left now is to create a new child theme by copying the files of existing child theme. I’m still concerned about how updates in original child theme is reflected in copied child theme. Keep track of the changes in original child theme and copy them again ?

Ask your hosting provider to get the details of FTP or you can browse the directory using CPanel if it is available to you. Just go and delete /home/content/65/8794565/html/wp-content/plugins/sydney-toolbox.

Please check with your hosting provider about how you can access the files and folders in your WordPress installation. Without that it seems to be impossible to solve this error by WordPress dashboard.

You can try reinstalling sydney toolbox plugin. The existing sydney toolbox plugin in your WordPress installation seems to have a PHP syntax error.
Just go to ftp and delete the folder /home/content/65/8794565/html/wp-content/plugins/sydney-toolbox. Later you can install the sydney toolbox plugin from your WordPress dashboard. I hope this will work for you.