The encrypted volume cannot be unlocked without the decryption key. It's impossible to access any data on this partition without proper authentication.
You will need to format (erase) the volume to be able to use it.
There is a disk formatting tool called "Disks" that can be used from a live session of Ubuntu. This will allow you to format the volume or the entire disk.
To use a live session of Ubuntu, boot from Ubuntu installation media and choose the "Try Ubuntu" option.
Then open the "Disks" application. To format the whole disk, select the disk on the left and choose the "Format Disk" option from the hamburger menu on the top right.
Since this device is "new to you", it is best practice to wipe all of the hard drives this way before installing an operating system. This will ensure that you don't inherit any malware or other software problems.
If you just wanted to format that partition, select the disk and sda3 and choose "Format Partition" underneath "Additional partition options".