Try directly assigning it. I’ve had more success with direct access request. I think it has to do with how Wix internally compiles the $w() selector. When you reference something selected, wix doesn’t always find what you selected the way you would think:
const opts = [ {label: ‘Brazil’ , value: ‘1’ }, {label: ‘Arg’ , value: ‘2’ } ];
$w( ‘#21’ ).options = opts;
$w( ‘#21’ ).selectedIndex = 0 ;