If the association of your gateway power outlet / module with HomeKit fails, proceed as follows:
Case 1: Installation already done under another iCloud account
Make sure that the installation has not already been completed using a different iCloud account, e.g. with the iPhone of a friend or family member.
If a friend or family member have already installed the products using their iPhone, they must send you an invitation so that you can share access to the products via the Apple Home app.

See How can I invite a user to share the control of my HomeKit home?" for more information.
Here's how to invite a user :
- Open the Home application
- Select the house whose access you wish to share
- Go to Home Settings
- Invite people
Once the invitation has been accepted, you can view the installation from Apple's App Maison.
Case 2: Setting up a first iCloud account
If it is the first iCloud account to access the installation, reboot the Wi-Fi and HomeKit settings stored in the gateway power outlet / module by doing a 10 seconds long press on the gateway power outlet button or gateway module configuration button.
See "How do I reset my Gateway and HomeKit?" for more information.
Gateway power outlet
Gateway module
Updated