The main administrator is being created while installing / registering a new shop.
In administration panel go to Configuration => Configure modules
Select Administrators module
Select Administrators
Only Super admin can add the new administrators.
Only the ordinary administrators can be used for WebAPI functions.
If a new administrator isn't set as Super admin:
Press and hold ctrl (cmd on Mac OS) button to select / deselect an option.
Press and hold shift button to to select / deselect all of options.
Select Permission groups
Press Add
Press and hold ctrl (cmd on Mac OS) button to select / deselect an option.
Press and hold shift button to to select / deselect all of options.