There's a few reasons you may not be able to create a folder:
If you use invalid characters, like < or /, in the folder name.
If you use certain reserved words in the name, like NULL, AUX, or other system terms.
You may not have the right permissions. If you're a member of a shared folder, you need a Contributor or a Manager role to create a folder.
If you can't perform an action, the action isn't available. For example, if you can't create a subfolder, the option to create a folder isn’t shown.