I need to download the macOS 10.15

Sounds like you downloaded the 10.15.7 update from Apple Support Downloads

you can only install these updates if you are already running an earlier version of macOS Catalina.

To upgrade to macOS Catalina you need to download and install the macOSCatalina full installer app

which can only be done through the App Store. The full installer app is called Install macOS Catalina.app

it is 8.26GBs and will install a full working version of macOS Catalina 10.15.7.

Apple removes previous versions of the full installers when the latest / last version is released that is

why there is no 10.15.

For macOS Catalina,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to the Download OS section and click on macOS Catalina.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS Catalina.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you want to or are able to install Big Sur, open Safari and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are offering 64bit updates.