The above image shows the gparted screen. I have installed kali linux in the extended partition. I unmounted the sda5 and turned swap off therefore, nothing is mount in the image. I also have a 20.00 GB unallocated space which I want to extend with the extended partition.


But when I try to Resize/Move the extended partition (like the above image) it doesn't provide the interface to slide the slider to increase the partition. My main target is to create a new child partition under the extended partition.
Any help will be appreciated. Thanks.

You cannot see the free space because it doesn't exist next to the extended partition. You have another partition (sda3) between the free space and the extended partition.
You need to move sda3 to the left to take up the free space, and create another free space at the right end of sda3 to create a new free space which touches the extended partition. Then you will be able to increase the extended partition, and the logical partition inside it (sda5).

Not worth the trouble - just take some space from sda3. You could even add the 20G to sda3 if you really feel the need.

Thank you it worked properly.