Jira Structure - Issue with "extend" generator

[Simran Brar]

Jira v7.13.5

I am trying to use "extend" to add linked/blocked issues (Stories). However I keep getting an error "You are about to create a generator. It will be impossible to add it to the structure because of the automation installed: Cannot add generators to dynamic content. You can drag-and-drop the item to a different location." Only query on this forum I gather suggests not having any other query highlighted; and it isn't in my case. Not sure how to get around. 

[Scott Hung]

You need to select the generator you are 'extending'.  see this reply:  https://community.atlassian.com/t5/Jira-questions/Structure-Error-quot-You-are-about-to-create-a-generator-It-will/qaq-p/982614