Confluence has been updated to version 6.15.9

Synchronization between two instances requires a Connection configured on both sides.
One side needs to initiate the connection and send an invitation to the partner (the Destination instance).
It means that the invitation includes pre-populated connection details such as Connection name and Destination URL.
The other side simply needs to accept the invitation and finish the configuration from their side.

Start synchronization with your partner

Initiate Connection and send an invitation to your partner.

Have an Invitation code? 

If you have the Invitation code - Accept Invitation.