10.13.6 to 10.15.3

That will be difficult now as the only full installer available via the App Store will download version 10.15.4.

Apple removes the previous full version installer when it updates to the next version of the current OS.

Why is 10.15.4 problematic for you.

Yeah, ok, you might have tried it before but it does work.

As I say open Safari, Firefox does not work with this link.

When the Mojave page of the App Store opens it should say 'Get',

if it says 'Open' then the App Store is detecting a partial copy of Install macOS Mojave.app

in your Applications folder, this will cause the error message you received.

Move it to the Trash, empty the Trash.

Connect your mac to your router using a cabled connection rather than WiFi.

Restart your mac, click on the link again and the App Store should now say 'Get'.

Do not use the mac while it is downloading, do not use any other devices that share the

same internet connection while the mac is downloading.

See the screenshot below and note the date and time, top right.

Well that is up to Protools, Mojave is still available for download.

Open Safari and click on the link below, do not use Firefox.

How to upgrade to macOS Mojave - Apple Support

Go to Section 4 and click on Get macOS Mojave

this will link you to the App Store Mojave download page.

It will download version 10.14.6.

Good, so now that Mojave is downloading, there is one thing you should do before installing it.

The Install macOS Mojave.app will download to your Applications folder. If when the download has completed

the Install app opens, just quit it. The Install macOS Mojave.app will self delete when the installation has completed

so it is a very good idea to make a copy go the Install macOS Mojave.app before installing it.

Make a copy and keep it safe on an external drive incase you need to re install at some point or have other macs you

would like to install macOS Mojave on. Saves having to download it again.

It is also possible to make a bootable USB stick installer using the Install macOS Mojave.app in the Applications folder,

and you can use that to reinstall Mojave or install on other macs.

How to create a bootable installer for macOS – Apple Support

Ok so that may be because you are using High Sierra and I am on Mojave,

the App Store looks different in Mojave.

Have you clicked on Download, and if so was progress made.