Have you read the instructions that pop up when you open the zip file?
For a child to find its parent, it must know 2 things a) the name of the folder containing the parent theme; b) the name of the parent theme as defined in the parent style.css file.
a) Inside the child theme zip , please open style.css file and find a directive which looks like
that is the url to the folder containing the parent theme. (capitalized/small letters are different, so Flagship is different than flagship.) The name of the folder that contains the parent theme must be correct here. If you renamed the theme folder, please make sure that child.css file shows the correct value for the name of the theme folder.
b) Inside the child theme zip , please open style.css file and also open the style.css file of the parent theme (you can view source to see it or look at the base of the parent theme folder). Inside the child theme style.css file, the directive "Template: " must have an identical value to the value of directive "Theme Name:" in parent theme's style.css
So if in your parent theme's style.css it shows "Theme Name: Flagship" or "Theme Name: I am Co0L" then your child theme's style.css must show "Template: Flagship" or "Template: I am Co0L". (capital/small letters are different, so Flagship is different than flagship.)