The use of the Singular / Plural feature is limited to certain fields. This feature can be applied to any field that corresponds to the Generic field for Author, Secondary Author, Tertiary Author, Subsidiary Author, Pages, and Cited Pages.
In your example the field you are using is Issue which not a field that can be used with the Singular / Plural feature.
You could instead enter into the Issue field nos. 2-3 so that it will appear in the document that way.
If you would like to see this feature in a future version of EndNote, You may want to submit a request for a product enhancement on our website.
https://community.endnote.com/t5/EndNote-Product-Suggestions/bd-p/en-suggest