Unallocated space for Ubuntu shown as "unusable" rather than free space

My motherboard is of Asus. I have Windows 7 installed already and I created 20 GB of unallocated space for the installation of Ubuntu in the C:/ drive.

According to the online tutorials, that space should be free space, but during the installation process that 20 GB is shown as unusable (see the next screenshot).

Proceeding further I also get the following warning:

You have a BIOS/CSM/legacy-mode installation of Windows using the Master Boot Record (MBR) partitioning system. This creates two issues you're encountering:

• You've booted the Ubuntu installer in EFI mode. If you were to finish installing Ubuntu in this way, you'd have Ubuntu installed in EFI mode and Windows in BIOS mode. Such installations can work, but they require jumping through extra hoops. It's much better to install Ubuntu in BIOS/CSM/legacy mode, the same as Windows. You can control the boot mode by selecting the correct boot option for the boot medium in the boot selection menu or by preparing a boot medium without EFI-mode support. This page of mine covers the issue of the problems mixing EFI-mode and BIOS-mode booting; however, it's written with the assumption of an existing EFI-mode installation and the risks of accidentally installing Linux in BIOS mode, rather than the other way around, so some details and procedures on that page don't apply to you.
• The MBR partitioning system supports a maximum of four "primary" partitions. To create more partitions, you must either delete one partition or convert one to "logical" form, which will enable you to create additional logical partitions. There are quite a few existing questions and answers on this site on this issue; see here for just one.

Overall, your best bet is probably to delete one partition or convert it to logical form, then boot the Ubuntu installer in BIOS mode to do the installation. An alternative would be to convert your Windows installation to EFI form, as described here, then install Ubuntu in EFI mode. This is likely to be riskier and harder than forcing a BIOS-mode install of Ubuntu, but would have the advantage that you could disable the CSM on your computer.