run Check Geometry and click Create Fluid Body Assembly.
This creates a model of parts that represent the various fluid cavities so you can make sure there are no extra holes or unexpected walls in your model. If this is a single flow path cavity the model it creates should consist of one part. If this part doesn't look like your expected flow path you have holes or extra walls.
If it let you add the inlet BC then likely there is a wall cutting your flow path (and creating a closed volume) and then a hole on the side where you want to put your outlet BC. Somehow you have a closed fluid volume that your inlet is part of, but it is not contiguous with your outlet.
Thanks Chris. I'll give that a shot and let you know how it turns out.