Syntax Error: AppleScript for Numbers

Edited my original post with a better revision.

What is happening is when you set the value of the first cell, it changes the selection range to just that one cell. When you try to do cell 2, there is no cell 2.

A change to your code so it would work would be like this:

tell application "Numbers"	
	activate
	tell the first table of the active sheet of document 1
		set myRange to selection range
		repeat with i from 1 to the count of cells of myRange
			set the value of cell i of myRange to (random number from 1 to 100)
		end repeat
	end tell
end tell

Note that you are assuming the selection range will be in the first table of the sheet. Another way to do it would be this:

tell application "Numbers" to tell front document
	tell active sheet
		tell (first table whose class of selection range is range)
			repeat with c in (get selection range)'s cells
				set value of c to (random number from 1 to 100)
			end repeat
		end tell
	end tell
end tell