CanalPlanAC has taken too long to do what you asked it, and has been stopped.
There are two reasons for this: you've stumbled across a genuine bug in the software or the server is very busy at the moment and has not been able to give you enough time.
In case this is a real bug it has been reported by email to the site maintainer. But as it may be just a busy machine you can try again.
If you keep ending up here, it probably is a real bug. In that case, press "Cancel" to go to the home page.
To allow things to quieten down, there will be a delay until the "try again" button appears.