I went to the ANSYS Help system, and found the equation for Norton so that I understood the creep model in another discussion.

ANSYS Help > Home > Mechancial APDL > Material Reference > Nonlinear Material Properties > Rate Dependent Plasticity > Creep

The Combined Time Hardening entry says only C1 > 0 and C5 > 0.  

Where did you obtain the creep constants?  What units of stress are required to use those constants?

It seems your solution begins, so you have some results.

Please reply with a snapshot of the graph of maximum displacement vs. time.