Hi @SalmonMan,
thank you for reaching out!
You realized that you can use symmetry constrain, and it’s good! 
At the first sight you have issues because of the too coarse mesh. The error message also suggests that we have the problems somewhere there:
In order to capture the geometry, a more refined mesh is needed.
I would first check the bounding box size and position. I think for simple flow visualization it is too big. Also, I’d put the object farther from the inlet.
Another important aspect is that the mesh element is preferably cubic, all edges are the same length,
In the area of the object, different refinements are needed to ensure that the mesh describes the geometry well.
You can read about or check different public projects to have the feeling.
If you’d like, I could make a draft mesh which you can use or improve further.
Don’t give up, CFD takes time to learn and more to excel.
Best regards,
János