Unable to edit custom field for Case in Application composer

Summary:

Content (please ensure you mask any confidential information):

I am trying to make some changes to a an existing custom field. I can add additional meaning but I want to create.

I can create new Meaning fields below:

but when i try an create new Case Type codes under the Look up type the icon is greyed out:

Is this because of the padlock icon against the Case?:

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):