This function has now been restored, so it was probably just a system issue.
According to the information on the AMEX transfer page, transfers of MR points to AC Aeroplan are temporarily unavailable both online and by phone. They are working to resolve the issue. If you need to book an award in the next few days, keep an eye out for updates.
I checked, and it looks like Chase is not affected. So if you need to transfer urgently, you can try using UR instead.
