I went to the URL you provided and there is no element with id='single-course-registration' on the page let alone an iframe. I found an anchor with an href containing 'single-course-registration' and I found two iframes. To click on the email link you need to find the element for the email link, which is NOT in an iframe. Once you find the element, click it. This will cause the iframe to become visible. You then need to find the iframe and switch to it. In comments:
// find the anchor element
// click the anchor element
// find the iframe element
// switch to the iframe element
at this point you can find elements on the iframe and interact with them. If you close the iframe you need to switch back to the default content before you attempt to interact with it.