In Sociocracy, is the facilitator allowed to interrupt someone?

If the facilitator is acting in facilitator role (and not in member role), then yes. Interrupting others might be necessary to keep time, clarify, provide guidance and emotional support. This can be done more effectively if it happens with respect and transparency.
If a circle member who is facilitator interrupts someone to say something from member voice, it will likely be seen as overstepping the role which undermines trust.