Hi, do you have an error log?
This seems to be caused by a line of code and a specific PHP version. You need to re-download the theme from Themeforest and manually update over FTP. Here is more info about updating over FTP https://archived.seventhqueen.com/kleo/article/updating-the-theme
You can update the whole theme folder or just wp-content/themes/kleo/lib/importer/import.php which is the file causing that error.
Let me know if you need more guidance.
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