I am receiving the following warning with Vivado Placer:
This error usually happens if the DSP is placed at the top of a column. In this case there are no physical connections to an adjacent DSP slice.
Another potential cause could be if you are trying to connect a PCOUT to a fabric component instead of another DSP Slice. The P ouptut should be used if you need to connect to fabric.
This is not a valid connection as the PCOUT is a dedicated cascade that can only connect to the PCIN of an adjacent DSP Slice.