Error when using DrawFormattedText in Psychtoolbox

I want some text to be displayed in the screen by using PsychToolbox (MatlabR2019b, Windows). I am using the "DrawFormattedText" function, but when I do I get an error that I cannot solve myself. Here is my simplified code:

function pruebahelp

backcolor            = [200 200 200];
res                  = [400 400];
[win, rect] = Screen('OpenWindow',0,backcolor,[0 0 res(1) res(2)])

Instr1 = ['Hello world'];

Instr = [Instr1];                       
DrawFormattedText(win, Instr, 'center', 'center',[0 0 0]);
Screen('Flip', win); 

pause(3)
sca

end

When running this code i get the following error:

% Error using Screen
% Usage:
% [normBoundsRect, offsetBoundsRect, textHeight, xAdvance] = Screen('TextBounds', windowPtr, text [,x] [,y] [,yPositionIsBaseline]
% [,swapTextDirection]);
% Error in DrawFormattedText (line 295)
%     theight = RectHeight(Screen('TextBounds', win, tstring));
% Error in pruebahelp (line 11)
% DrawFormattedText(win, Instr, 'center', 'center',[0 0 0]);

I would appreciate some help!

[Molly]

I also need help with this question!