You can't use JavaScript variables or functions directly within your html markup in that manner. What you can do is define your iframe first and then set its source from JavaScript:
<iframe id="showskill" scrolling="yes" height="350" width ="350" src=""></iframe>
<script>
document.getElementById("showskill").src =
"http://localhost/POSkill/skillshow.aspx?user_id="+ ReturnURL();
</script>
There are several other ways to achieve something similar, but I don't really want to go through them all when I'm not sure quite what your context is.
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