I see "Site Factory Maintenance in Progress" while configuring Site Factory configurations or adding any domains.
If there is no active maintenance occurring, contact Acquia Support via a support ticket to clear the Site Factory Maintenance Form.
There are various reasons this popup will appear. Two most common causes are:
1. If any active maintenance is going on, then Maintenance mode will be enabled.
2. In case of failure in any deployment or if any task has been prematurely killed, then Maintenance mode gets activated and the popup will be displayed.