I am not able to reorder activities as there's no "4way arrow" button

Activity Center
 
 
If you don't see the "4way arrow" button that allows to reposition activities/sub-activities then you need to switch to the hierarchical view using the button in the top right:
 
 
Q. There is the "4way arrow" button however I cannot make a subtask a top level task or reorder top level activities.
A. Please make sure that the sorting mode for activities is WBS: