If you are on Windows (not Linux, not MacOS) so try to download/buy from the market tab in Metatrader.
There are some information below which may be useful for you -
Tips for Purchasing a Product on the Market. Step-By-Step Guide - the article
how can i down load an expert advisor for demo testing from the markethttps://www.mql5.com/en/articles/586
Market Tab Error, show a blank page and missing optionhttps://www.mql5.com/en/forum/170760#comment_4177231