Enable Custom List Settings In Org

From  Spring '18 release, By Default Custom List Settings Are Greyed Out,
If You Are Trying to create List Type No way to select List Type By Default Is Shows Only Hierarchy Type.

To Enable List Type Custom Settings, In Your Org
Path:

 Setup > Schema Settings > Manage List Custom Settings Type

Enable Manage List Custom Settings Type.

Once You Enabled The Toggle you are able to create list type Custom Settings in Your Org.

Let us know if you have any queries!

Happy Learning!!!