Create folder in Pictures

Is the Pictures folder open in the Finder and the front most Finder window when you try that?

Then check the permissions on your Pictures folder.

• Select the Pictures folder itself in your home folder.
• Enter the key combination ⌘i to open the Info. At the bottom of the Info you will the permissions. Do you have read and write permissions?

You can create subfolders in the Pictures folder like in any other folder.

Open the Pictures folder in the Finder and use the command "File > New Folder ⇧⌘N"