Why doesn't a feature allow opt-in when it's not enabled? How do I enable it?

An always-enabled feature won't be enabled until the parent of the feature is also enabled.

Therefore, even though the Allows Opt-In column of that feature doesn't show a check mark indicating that it's always enabled, its Enabled column may not show a check mark. In such cases, use the Go to icon in the Enabled column to enable the parent feature. Once the parent is enabled, a check mark automatically appears in the Enabled column, for that feature.