IDA can only display functions in graphical mode if they have a well-defined start and end address. Normally, the start and end address are determined from the disassembly - the start address is the target address of
call instructions, the end address is where the code has a
ret instruction (*).
In your case, IDA failed to make a function from your code, probably because there is no call to
start, and maybe there's not even a
ret at the end. You can try to fix that by moving the cursor to the
start label and pressing the
P key. Once your code is defined as a function, the space key should switch between graphical view and assembly view just like your tutorial says.
(*) Actually, it's a bit more complicated; some parts of the function may be behind the
ret; but IDA detects this from jumps within the function.