Login to your Supliful account.

On the top, click Stores.

On the Stores page, under Add new store click on the WooCommerce tab:

In the pop-up window enter WooCommere store details and click Connect.
Make sure, your WooCommerce store name does not have "/Shop" at the end.
Like this - wordpresshostname/shop. This does not allow REST API connection.

Just add wordpresshostname:

You will be taken to the WooCommerce connection page. Make sure you are logged in as the correct user and click Approve:

Your newly connected WooCommerce store will immediately appear on your Supliful Stores page and you can start publishing products right away.