I created a Drupal 8 module for creating a custom field. I wrote the code basing on the documentation given in Create a custom field widget, Create a custom field formatter, and Create a custom field type.
I created the .info.yml file, three directories src/Plugin/Field (FieldFormatter, FieldType, and FieldWidget) where I put the files described in those documentation guides.
I could see the custom field in the field list, but it is shown as disabled.
Why does this happen?