This error occurs if the PlanAhead project and the actual directory structure are out of sync, in that the project (and data files) indicates no run, but the synth_1 folder is on disk.
The message indicates that the directory could not be created because there is already a directorywith that name for synthesis process(directory <path>/synth_1) and the tool does not expect tofind it there.
To fix the problem, you will need to rename or remove this directory and re-run the synthesis process.
In PlanAhead 13.3, the message has been changed to the following:
[Runs-51] Unable to write to run directory <path>/synth_1. Please manually rename or remove this directory and try this operation again.