Why is WooCommerce Showing Warnings in my Backend, and on the Website?

Check how to disable debug mode in WooCommerce.

This error usually happens, if WooCommerce's Debugging has been activated. This is never a good idea to have activated on a live-setup because it slows down pages, and potentially shows error messages in the frontend.

The sync issue happens if WooCommerce encounters an error while generating the product-data for the import. 

This can be solved by deactivating Debug Mode either through the plugin or directly in the WooCommerce files.

Using the Plugin

If you are using Clerk.io's v2.2.4 or later, Debugging can be disabled directly from the Plugin page in your WooCommerce backend.

Go to Clerk -> Clerk Settings -> Debug Guide and click Disable Debug Mode:

Or Directly in the Files

Debug Mode can also be deactivated through FTP, in wp-config.php by setting define(WP_DEBUG', false). 

This guide explains it in more detail:
https://docs.woocommerce.com/document/woocommerce-product-search/api/debugging/

And this guide explains an alternate workaround:
https://aristath.github.io/blog/wp-hide-php-errors

Errors will still be logged even when not in Debug Mode, but they will be logged to a file instead.