"Not root file system"

I try to replace an existing Linux installment with another one, Zorin Lite, on a dual boot PC. After selecting the Linux partition /dev/sda3 (ext3) for installment, I get a message "not root file system" (see the attached image). I did an online search and learned that I need to remove the partition and create a new one. On the creating a new partition panel, I see something like "start space" and "end space" and the selectable devices are quite different than the existing device name /dev/sda1, /dev/sda2, ... . I am wondering whether I am in trouble or not although the Windows still work from the dual boot.

How to go from there?
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