it looks like you only downloaded an upgrade to macOS Sierra.
Welcome and thank you for your post.
Just for further clarification:
Where are you downloading macOS Sierra and macOS Mojave from? This is important.
(I noticed you missed out macOS High Sierra which comes between the other two).
And also: what year and Model of Mac have you got because it may be that El Capitan is as far as you can go.
Shower us with details please. It makes life easier and the solution comes sooner.
Ian
From the image you posted, it looks like you only downloaded an upgrade to macOS Sierra.
See this Apple Support document for links to older versions of macOS available in the macOS App Store.
How to download and install macOS - Apple Support
Download and install current or previous versions of the Mac operating system on compatible Mac computers.support.apple.com