what is wrong or what I should do to improve[?]

make the array be in range.

Perhaps you should...

so now you need to figure out why it doesn't have a value. The only way to not have an initialized value is to create a new variable and to not put anything into it.

But the compiler is telling you that this is not the case, so something is not adding up to where you expect it to be.

It could be a number of things:
-- (i-1) is evaluating to a different value than you think it does
-- array itself is different, perhaps because of scoping rules
--- or because it's been cleared since last use.

use of put statements could help you trace through what variable contains what value, at what time in your program.

%% Snake game : Szymon Wyborny
View.Set ("graphics:600,600")
View.Set ("offscreenonly")
colour (white)
colorback (black)
var tempx, tempy : int := -100
var x, y : int := 0
var x2, y2 : int := 0
var playagain : string (1)
var font1 : int
var scoreint : int := 00
var scorestring : string
var k := 100
var i : int := 0

font1 := Font.New ("Dauphin:17")

var z : string (1)
var foodx, foody : int

randint (foodx, 24, 576)
randint (foody, 24, 576)
loop
if foodx mod 12 not= 0 then
randint (foodx, 20, 568)
end if
if foody mod 12 not= 0 then
randint (foody, 20, 568)
end if
exit when foodx mod 12 = 0 and foody mod 12 = 0
end loop

drawfill (0, 0, black, black)
drawfillbox (12, 12, 588, 576, white)
View.Update


loop
var ary, arx : array 0 .. k of int
k := k + 1
i := i + 1
put "Your score: ", scoreint
% Font.Draw ("Your score: ", 100, 580, font1, black)
%
% scorestring := intstr (scoreint)
% Font.Draw (scorestring, 210, 580, font1, black)

if hasch then
getch (z)
locate (1, 1)
if ord (z) = 200 and y not= -12 and y not= 12 then
% put "Up Arrow Key"
y := 12
x := 0
end if
if ord (z) = 203 and x not= 12 and x not= -12 then
% put "Left Arrow Key"
x := -12
y := 0
end if
if ord (z) = 205 and x not= -12 and x not= 12 then
% put "Right Arrow Key"
x := 12
y := 0
end if
if ord (z) = 208 and y not= 12 and y not= -12 then
% put "Down Arrow Key"
y := -12
x := 0
end if
end if
arx (scoreint) := x2
ary (scoreint) := y2
drawfillbox (12, 12, 588, 576, white)
drawfillbox (foodx, foody, foodx + 10, foody + 10, black)
drawfillbox (foodx + 2, foody + 2, foodx + 8, foody + 8, yellow)
View.Update


drawfillbox (60 + x2, 60 + y2, 70 + x2, 70 + y2, black)
drawfillbox (62 + x2, 62 + y2, 68 + x2, 68 + y2, green)

tempx := x2
tempy := y2

View.Update


x2 := x2 + x
y2 := y2 + y

% GROWTH
for s : 1 .. scoreint

drawfillbox (60 + arx (scoreint-1), 60 + ary (scoreint-1), 70 + arx (scoreint-1), 70 + ary (scoreint-1), red)
end for

View.Update
if 60 + x2 = foodx and 60 + y2 = foody then
scoreint := scoreint + 1
randint (foodx, 24, 568)
randint (foody, 24, 568)
loop
if foodx mod 12 not= 0 then
randint (foodx, 20, 568)
end if
if foody mod 12 not= 0 then
randint (foody, 20, 568)
end if
exit when foodx mod 12 = 0 and foody mod 12 = 0
end loop
end if






if 60 + x2 > 576 or 60 + x2 < 12 or 60 + y2 >= 576 or 60 + y2 < 12 then
locate (1, 23)
put "GAME OVER"
locate (1, 37)
put "Press ANY key to play again"
View.Update
getch (playagain)
cls
scoreint := 0
x := 0
y := 0
x2 := 0
y2 := 0
drawfillbox (12, 12, 588, 576, white)
View.Update
end if
locate (1, 1)
delay (60)

end loop

drawfillbox (60 + arx (scoreint-1), 60 + ary (scoreint-1), 70 + arx (scoreint-1), 70 + ary (scoreint-1), red)