Merchants can upload entire collections of products to their newly-created bundles. This allows you to bulk-import products if you intend on your Bundle having a wide range of options.
Sometimes, this process may fail - resulting in a warning being displayed (or similar).
When uploading, you may encounter an error. There's a couple of reasons why this feature might fail.
Quantity of Products included in the Bundle
Currently, Shopify displays a timeout on the request, which is around 30 seconds. If the process takes longer than this specified time, it will fail, and the process will stop and have to be re-attempted. Unfortunately, there is currently no work around for this, as it is a platform-implemented restriction. To work around this, you could make a smaller collection, and attempt to import again or manually import products individually, instead.
If there is a storefront password present on the store, this will prevent the items from being imported.
You will need to temporarily remove the storefront password, import the collection, then reinstate the password.