Since Version 13 (via #11436), assets can be downloaded if they are available on GitHub releases or http://assets.frappeframework.com. This means Frappe tries downloading assets first to avoid having to build them itself. These errors come up if you’re running on an untagged commit of a stable branch, ie only version-13-beta at the moment, or a custom branch (not develop).
There’s another scenario where your clones might’ve in the middle of releases, or that commits are added over the stable branches without releases that can cause this error to come up. However, this is a harmless error and just means your server will have to build its own assets instead of having to depend on pre-built bundles.