How to git add a whole folder

My guess is that you are trying to add folder while you already are in folder.

$ cd my_folder
$ git init
$ git add my_folder # this is not going to work

Instead, add everything in the folder, rather than the folder itself:

$ cd my_folder
$ git init
$ git add .

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To your other question, adding whole folders is fine, but only relevant when adding sub-folders. Again, you can't git add the folder that is your repository (my_folder above).

$ cd my_folder
$ ls
my_subfolder  other_things
$ git add my_subfolder # this is fine

The usual way to add everything in your working tree to your repo is git add ..

If the folder contains space you need to enclose the name with quotation marks:

git add "New Folder/"

You need to first check if you have added that folder in .gitignore file.

If not, then just do this.

git add --all 

or

git add .

I think your folder is not under git version control, or you ignored it.
try to run git add -A to add your folder