Tampermonkey how to fix the error-enable js

I try to "throw" a small browser script for Tampermonkey on JS, when running on the destination site, I get a page message instead of the html code that for the browser to work correctly, you need to turn on js, but it is turned on there... How can I try to fix it? Thanks!

window.onload = function(){
    setInterval(function() {
        var source = document.body.innerHTML;
        var str_before = '...';
        var str_after = '...';
        var changed = source.split(str_before).join(str_after);
        document.body.innerHTML = changed;
        alert(changed);
    }, 1000);
};