Assuming you can boot from a USB stick with a current version of the Ubuntu installation ISO (20.04 is an excellent choice, as noted), select the "Try Ubuntu" option from the initial startup screen, and run the Disks tool (this is in fact the superb GParted program) before you try to install.
Disks should list your 1 TB HDD on the left column; click on this item and you'll see a graphical representation of the partitions on your 1 TB HDD.
Because of these existing Windows partitions, this 1 TB HDD probably has very little or no free space available, which is why the Ubuntu installer won't install on it. Note that "free space," from the installer's perspective, is space on your HDD that isn't partitioned. So you may have a large Windows partition with lots of "free space" but since this space is already allocated to the Windows partition, the installer can't use it.
You can delete all of the partitions on this 1 TB volume to free up space for your Ubuntu installation, but that means that you'll lose all of the data and software on the 1 TB HDD. Before you do that,
- Make sure you've backed up any irreplaceable data files on the 1 TB HDD, because once you delete the partitions, that data is gone forever.
- Ensure that you can reach the Internet when you Try Ubuntu. You can test this by opening a browser and trying to reach, say, Google or Stack Exchange. Installing Ubuntu to an internal HDD is much easier if you can download the most recent updates during the installation process.
If you're ready to clear off the 1 TB HDD, Disks will let you select and delete each existing partition. Note that none of the commands you issue in the Disks program will be applied until you confirm that you want to proceed; then all of them will be executed in the sequence you specified, as a batch job.
After you've deleted all the partitions on the 1 TB HDD, you can exit Disks and run the Install Ubuntu program on the desktop, or reboot from the USB stick and select the Install Ubuntu option when it boots up. Now the installer should see all of the free space on your 1 TB HDD, and install accordingly.
Note that you can also use Disks to resize partitions and set up a dual-boot system if you still want to use Windows, or even install Ubuntu as a program you run after you boot Windows. These options are more complex, and not what you indicated you wanted to do, but may be worth considering if you want to preserve your Windows setup for now.
If, for any reason, Disks reports errors when it tries to delete or resize a partition, this means your HDD has some serious problems and should be replaced.
