Ankit, you have very nicely explained the difference between SEO and SEM. I understood them to be part of it each other, but never gave it a proper thought. Search Engine Optimization (SEO) is a technique that helps raise up our (search) rank bring more traffic to our website. While Search Engine Marketing (SEM) is a larger concept that includes SEO as well. Internet Ads are an important component of SEM.
I just tried and its working fine from my end. When clicking on the link, you should be taken to the SEMrush page where you should be able to grab SEMrush pro account for free for the next 14 days worth $47. Please clear cache for your browser and try then. Please let me know if you still face any issue in claiming the 14 days SEMrush pro account for free.