The installer cannot detect the computer harddisk, it can only detect space in usb. My harddisk has space with an empty partition. There is both windows and Ubuntu installed in the disk.
An (what you call) empty partition is NOT free space on a disk. Free space is unpartitioned space.
The installer can not read minds and thus the installer does not know that it has to see that partition as unused space.
So remove unsused partitions and see what the installer tells you.