The is the function I am using to call the API
 private String getDepartmentName(Integer department) {
        final String[] return_val = new String[1];
        return_val[0] = null;
        managerLauncherApi.getDepartmentName(department, new Callback<List<ObjectDepartment>>() {
            @Override
            public void onResponse(@NonNull Call<List<ObjectDepartment>> call, @NonNull Response<List<ObjectDepartment>> response) {
                List<ObjectDepartment> JsonList = response.body();
                for (int i = 0; i < requireNonNull(JsonList).size(); i++) {
                    objectDepartment = new ObjectDepartment(
                            JsonList.get(i).getId(),
                            JsonList.get(i).getDepartment()
                    );
                }
                return_val[0] = objectDepartment.getDepartment();
            }
            @Override
            public void onFailure(@NonNull Call<List<ObjectDepartment>> call, @NonNull Throwable t) {
                return_val[0] = "Department not found";
            }
        });
        return return_val[0];
    }

When I debug it, it goes from the managerLAuncherApi line to the return line at the bottom, skipping everything in between.

The request I make in Postman: http://phoenixhospman.com/api/launcher/getDepartmentName/5

And the result from Postman to the request: [ { "id": 5, "department": "Operations" } ]