"This selected collection might contain products that are not publically available. Products must be available in the sales channel "Online Store" in order to be included in the feed. The selected collection has currently xxx publically available products."
Regarding the message in red - it's actually not an error - it's just an alert to make you aware that the collection you have chosen might contain products that are not publicly available and thus won't be synced.
You need to check the "missing" items if they are available online. They need to be active and managed for the Online store channel on your Shopify admin page.
Hint: You could try to select all products on your Shopify backend, choose 'More actions', and then click on 'Add available channel(s)'.
