Solution
The VIs from the LabVIEW SystemLink Toolkit were primarily designed to work with older SystemLink products, like SystemLink Server (SLS). Although some VIs may still work with SLE, each VI is ultimately using HTTP(S), and hence, will attempt to use hard-coded API URL. Many of these URLs will be different between SLS and SLE.
The List Workspaces and Use Workspace VIs in particular will not work with SLE. Instead, you are required to build your own HTTP(S) request using the GET VI. The required URL, parameters and body can be found by navigating to the SLE Swagger UI.