When I click add to cart button in the bundle widget, I see a message that one of the products is not available. What to do?
If you set the app to hide unavailable variants, then please make sure that the selected variants are in stock.
If the issue persists, then it could be that you modified your product variants for a few of your products, which caused Shopify to change the variant ids.
Now, the bundle is configured with old variant ids which don't exist anymore. This is why you can't add the bundle to the cart.

To resolve this issue, edit your bundle, select the products again, add them to the bundle and save the bundle.
This will update variant ids and you will again be able to add the bundle to the cart.