Is this SharePoint online or on-prem? If online, the REST query will probably not work. If on-prem, you should have your SharePoint admin take a look at the logs to determine the issue. However, since this is a simple equivalence query you would not need to use REST at all. Add a list data connection to your list, but on the final dialog, deselect the checkbox to query for data on form open. Then, in your form, at the same point you were executing your REST query, set the list query field to the value you need, and then run the list query. The list query will only return data that matches on the query field.
Hilary Stoupa
