First attach the javascript function to your checkbox
chk.Attributes.Add("onclick", "enableDeleteButton();");
This will call function enableDeleteButton everytime you chick on checkbox. Also below is the function you can use:
function enableDeleteButton() {
debugger;
var repeater = document.getElementById('repeater');
var delbutton = document.getElementById('<%=btnDelete.ClientID%>');
delbutton.style.display = 'none';
var inputs = repeater.getElementsByTagName('input');
for (var i = 0; i < inputs.length; i++) {
if (inputs[i].type == "checkbox" && inputs[i].checked == true)
delbutton.style.display = '';
}
}
And in page load make delete button visible false by adding below code:
btnDelete.Style.Add("display", "none");
[Do not use btn.Visible = False as it will not find by javascript]
Let me know if need any help.