Upgrading to Catalina from Sierra

In fact, additional scratch space is needed during the install, people have found empirically that 30+ Gb free space is more realistic for a successful upgrade. The consequence of running out of room during the installation of a new operating system could be that it reverts back to what you have, or it could leave you with an unbootable computer. Since the latter is very serious, be sure to leave PLENTY of free disk space before you try to upgrade. Obtain at least two external drives, use one for a reliable backup of everything, and the second to store files you are moving off your internal drive to make more than 30 GB free space.

The installation app required for macOS Catalina is 8.26GBs in size, the information on the App Store is wrong.

When you have downloaded the install app you will have less than 8GBs of free space on your mac

and you will not be able to install Catalina.

Before you do anything you need to create more free space on your mac.

It is generally accepted not to let your mac run below 20GBs of free disk space,

the mac needs room to do its housekeeping and it is always moving files around.

If you run your mac too low, it will give you a warning at 10GBs or lower, if it gets any lower

it could lock up and become unusable.

Copy any files and folders you do not need on a daily basis over to an external drive.

When they have copied them over successfully then you can delete them from your mac.