I set the partition space for Ubuntu at 15 GB, but after installing programs, it appeared a pop up saying that the space was almost finished. If I go on Disk, I see "Extended Partition Partition 3" as 15 GB, and under it two squares, one with "Filesystem Partition 5" with 6.7 GB and the other one with "Swap Partition 6" with 8.3 GB and next to them, the free space left. Why can't I have the 15 GB space I set before?
You need to either shrink the swap partition or make more room for the other partition from somewhere else. 15gb is really small for a full install, though. If you never use hibernate and you have plenty of ram you can delete the swap partition and enlarge the other partition to fill the extra space if you want.