I guess so. 4-to-1 MUX is so small that it's just composed of several gates.
The room for adding more optimization device is 0.3*(cell area) which is also small.
Could you tell me which the stage is when the error occurs ? floorplanning ? powerplanning ? placement ? CTS ? routing ? manufacturing ?
To be more specifically, it's clues that we have to know which devices may be added.
Once the optimized device is supposed to add into the design, the remaining room is seemed to be insufficient.