If you're trying to enable Momentum (or another Safari mobile extension) on iPhone or iPad, but it's not responding, you've come to the right place.

This happens your device's Screen time Content & Privacy Restrictions are turned on. To fix this, simply turn those restrictions off.

Turning off Content & Privacy Restrictions

On your device, navigate to ⚙️ Settings > Screen Time > Content & Privacy Restrictions and then tap the switch next to Content & Privacy Restrictions. When you revisit your Extensions settings you should now be able to enable Momentum (and any other Safari mobile extensions).