Why does the Linux installer say I don't have enough space?

The total disk size isn't necessarily the space available to Linux. The installer may be looking at one small partition while the rest of the disk belongs to another partition or is unallocated.

From a live Linux session, run lsblk in a terminal and check the disk and partition sizes. It would also help to share the partition layout shown by the installer. Leave out any personal labels or other information you don't want public. Don't merge or delete anything until you know what each partition contains.

If the large partition is Windows and you want to keep it, you may need to shrink it to create unallocated space for Linux. Free space inside the Windows partition isn't the same thing as unallocated disk space.

Back up anything you need first, then shrink the Windows partition and check that the installer sees the newly unallocated space. The exact partitions Linux needs depend on the installer and boot setup, so check its choices before applying changes.

Are you trying to install Linux, or download something on a Linux system that's already running? Those are different problems. If Linux is already installed, its root partition could be full even though another partition has plenty of room.

Check where the download is being saved and how much free space that filesystem has. A separate empty data partition won't help a download going to a full root partition unless you save it there instead.

GParted can show the partition layout graphically and can resize partitions, but resizing isn't a substitute for a backup. Whether you can expand a Linux partition depends on where the free or unallocated space sits on the disk. Post the layout before making changes, especially if another operating system is on there.