I am not able to run the Arc Flash Simulation it is giving me the error:

" Calculation Error: Memory mapped file for source contribution has insufficient size. The file size can be increased by setting the preferences entry No. of Upstream Contributing Branches to a larger number (Max=999)."

However ETAP is able to run the short circuit simulations.

Any solution to this problem??

The default setting in Preferences is 3. The program looks for the number of upstream branch circuits that may contribute to the arc fault current. Check to make sure it's not set for zero or a large number. I found setting to a large numbers (50) eats up allot of memory. Unless you have a large number of motor fault current contributions on branch circuits setting to 3 works fine. It could also be caused by having another program running in the back ground that is taking up memory. I found that if you have Excel work sheet open with arc flash data from the report manager and run another AF study will cause error message. What happens is Etap report manager is trying to write to a open file being used by Excel.

Probably this is the option you talking about. The problem is solved the solution is
Tools>Preference>Arc Flash> No. of Upstream Contributing Branches

Change the number and it worked..

Yep that's the option.