I’m not sure if this is the issue David is referring to, but on the page Essential Templates Pack | RapidWeaver Elements , when I click on the “Open in Elements” button (top right), the address (elemetsapp://elements-library-template-pack-01) is incorrect (the letter n is missing). This is what causes the error message in Safari.
That was the issue. But selecting the marketplace from the app rather that using the link from the email promotion did the trick. I was able to purchase the essential elements pack.