What does "Cannot rebuild" mean on a snapshot?

That snapshot stores only its changes, and the full copy it builds on has been deleted. It cannot be opened or restored from.

Snapshots that are not marked Full are differences measured against a full copy. Delete that full copy and its differences have nothing to measure against, so the app marks them in red rather than letting you open one and discover it rebuilds into a fraction of your account.

The fix is simply to use a different snapshot. To avoid it altogether, leave the Full ones alone when you are tidying the list — they are the ones others depend on.