Before connecting your WooCommerce account to Sellbrite, first make sure that you configure your WooCommerce store and generate your API keys.
If you're encountering issues getting connected and you have plugins enabled within your Wordpress setup (which you likely do), they can sometimes interfere with connecting third party software applications like Sellbrite. Most merchants who have trouble connecting their WooCommerce channel to Sellbrite is because of one or more plugins.
Test this by disabling your plugins one-by-one in your Wordpress site. As you disable each one, click the connection button in your Sellbrite settings to test if it connects. If you're able to connect, you'll know which plugin is causing the problem: https://app.sellbrite.com/settings/channels
Additionally, make sure that you activate your Legacy REST API on WooCommerce. This is a common error that merchants get with WooCommerce that prevents them from connecting their accounts. Check out this article from WooCommerce to learn how to activate the Legacy REST API (go to the section titled, "Enable legacy REST API").
Another common reason for connectivity issues is your specific server settings. Make sure that you give the proper read/write permissions and test out the accessibility for third party apps, as well as checking the script execution time.