In the JIRA REST API documentation for the endpoint api/2/field/{fieldKey}/option (URL: https://docs.atlassian.com/jira/REST/cloud/#api/2/field/{fieldKey}/option) says

This resource represents the options for a select list issue field on an issue.

It is not immediate to understand that this is actually referring to a kind of field that can only be used by add-ons:

• https://developer.atlassian.com/static/connect/docs/latest/modules/jira/issue-field.html
• https://developer.atlassian.com/display/jiracloud/Guide+-+Using+the+Issue+Field+module

Also, expanding the methods for this resource there is a text that seems wrong:

Only JIRA administrators or the add-ons that provided the field may use this method.

It doesn't seem to be possible for JIRA administrators to call those endpoints.