In order to format the drive that has your OS on it we will need to boot from either a live disk or the Pop Recovery partition.
NOTE Formatting the entire OS drive (including the Recovery partition) requires booting from a live disk or another drive.
The Disks application can be used to format the drive to remove any personal data. Disks comes pre-installed on Pop!_OS and Ubuntu.
In this screenshot the drive is a USB Flash Drive, though the concept is the same for any drive, including the drive where your OS is installed. First left-click on the drive on the left side then left-click on the top right menu with three dots (three lines on older releases).
Now select the Format Disk… option then make sure to have the ‘Compatible with modern systems and hard drives > 2TB (GPT)’ selected (you can use either the Quick option or the Slow option).