+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
Just spoke with CIC on phone and its the issue at there end.
They asked me to try it again tonight or tomorrow morning.
Hope it helps!
 
Tweet from IRCC :

"The ePayment functionality is experiencing technical issues. This is being examined and we will provide an update once available. Thank you for your patience. "