Based on the small dataset you included, I believe the issue is Site2. Not all of the time points were observed AND not all of the positions were observed with Site2. Your model includes the Position*Site interaction and the Site*Time Point interaction. You cannot estimate all of the levels of those interactions because you do not have enough data (specifically, the missing combinations with Site2).
If you hide and exclude the Site2 data from the analysis, the non-estimable terms go away. Alternatively, you could remove the Position*Site interaction and the Site*Time Point interaction from the model.
Dan Obermiller