Final Cut Pro Trial can't be installed on this disk. macOS version 10.15.6 or later is required

Go to System Preferences to the Software Update pane. It should find the upgrade for you. Before you do that make a complete backup of your system using Time Machine or cloning software like Carbon Copy Clone or SuperDuper!

The image shows that you still have the old system. I think I see the installer behind it. It looks like the installion of the upgrade hasn't finished.

What OS are you using exactly? Is it 10.15.6 or later?

You need to update the operating system. The current OS is 11.1.

Look at the installer it will tell you what’s going on. The system will reboot when the installation is complete.

Go to the menu Apple > About this Mac and either post a screen shot or post what About this Mac says.

Using either method, make sure you obscure your serial number.

MtD

It’sa very small system drive. I would recommend you move as much content as you can onto an external drive.