When in the request or request item form, clicking on the reference does nothing. It is expected to open a popup to preview the records and select from there.
On the form layout, customer might be using a custom Variable Editor to show the Variable editor. However, it is important that the UI Macro used in that Formatter, should exist in the instance. Usually the UI macro used in the sc_request and sc_req_item table is com.glideapp.questionset.DefaultQuestionEditor. If for any reason, this Macro is unavailable on the instance, the above issue will occur, along with a browser console error will also be observed as :
Uncaught ReferenceError: ServiceCatalogForm17 is not defined
Below is the screenshot of the error on console:
Customers either can remove the UI Formatter from the form layout, or they can import the missing UI macro to the instance.