The question is did it fail to download, or is the activation key a problem?
If it is the former, all versions of eM Client are available to download from the Release History. See if that helps.
If it is the latter, from the error you have given it appears that you have already registered a Free License for that email address. If you have lost the activation key, you can have it resent to you at https://www.emclient.com/lost-activation-key
Please remember that it is not just your opinion of the cost (£30) of the Pro License that determines if you need to purchase it or not. If you want to use more than 2 email accounts, or you are using the application for business then you will need to purchase a Pro License.