Can't install Catalina

Look inside your Applications folder for Install macOS Catalina.app, if it has downloaded correctly it should 8.23GBs in size.

If it is double-click on it to start the installation procedure. The Install macOS Catalina.app self deletes after the installation has completed, so before starting the installation procedure make a copy of it and keep it safe on an external drive or USB stick or similar.

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.

If you are considering upgrading to Catalina be warned that it does 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/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

I always recommend backing up your data, preferably with a clone before installing a new operating system.

https://discussions.apple.com/docs/DOC-1992

And yes, Catalina, you are best waiting until all your applications are 64 bit. Otherwise you'll need to double partition your Mac for some applications and dual boot with the startup manager.

As I said it will be in your Application folder, the Launchpad is just a representation of the apps in your Applications folder.

Make a full back up of your mac before attempting the installation.

Open Disk Utility from your Applications/ Utilities folder and run First Aid on the Disk.

If any errors are found then boot to your Recovery HD, Restart while pressing and holding the Command and R keys,

run Disk Utility First Aid from there until you receive no errors. Restart your mac and run the installer when you are ready.

Also make sure you have enough free space for the installation, at least 30GBs free would be recommended.

Is there a compelling reason for upgrading to Catalina, Mojave is a warmer, friendlier place.

Which printer model is it? You might want to check the open source alternatives*:

http://www.macmaps.com/printersx.html

* Links to my pages may give me compensation.