If you are using Bootstrap Modal then be sure that Model tabindex=-1 is removed.

 $('#targetId').select2({
        width: '100%',
        dropdownParent: $("#myModal")
    })

Using Select2 in a Bootstrap Modal/PopUp If you are using Bootstrap Modal then be sure that Model tabindex=-1 is removed.

$('#targetId').select2({ width: '100%', dropdownParent: $("#myModal") })