Unable to Change Item structure from Primary to Other in Work DefInition

Summary

For an ATO Model Unable to Change Item structure from Primary to Other in Work DefInition

Content (please ensure you mask any confidential information):

I am unable to change the Item structure in the work definition from primary to another Value say "Manufacturing". The Item has two Item structures Primary and Manufacturing. While creating work definition, Item structure is defaulted to primary and non editable afterwards.

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

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