Casio 7400gII

[klesl]

Well, you are displaying HELLO and clearing the HELLO during one loop, so the results is blank screen with displayed busy indicator (square at the top right corner), because the disaplaying HELLO and clearing it runs practially in the same moment.
Unfortunately there is no Pause command in current Casio Basic, so you need to insert between locate and clrtext some loop, e.g. summation, factorial, product, addition, substraction, factorization, to see the "HELLO" for certain time before clearing it.