Can't download Catalina

So I have a 2017 MacBook Pro. I had been having hard drive problems lately, so I just started running off an external hard drive to get me by. I downloaded macOS through the recovery screen, as that was all I could reach, and it downloaded Sierra(10.2.6 specifically). Before my hard drive dumped I was running Mojave, and because of this, my computer won't let me restore to my backup unless I have Mojave or later downloaded.

I can find Catalina in the App Store.

But when I click download, it just goes to this and stays like that.

What should I check out to get this to work? Thanks in advance for any help.