Thanks for posting your database. In your specific case the issue was caused by the following:
In version 3 of Path which you are currently running, the number of simulation cells to run is calculated as the total area set by the user in ac divided by the resolution set by the user. Your model uses a resolution of 200*200 m or roughly 10 ac. The number of simulation cells is calculated as an integer so anything less than 5 ac in size will have a calculated number of simulation cells of zero and will result in the message you describe.
In Path version 4 the user sets the total area and the number of cells rather than the resolution. The user interface will not allow you to set the number cells to a value less than 1 so you should not encounter this problem. In the current beta release of version 4, when you upgrade your data from the previous version it will calculate the number of simulation cells based on the formula described above and you may end up with scenarios with zero simulation cells. This is something we will fix in the full release of version 4.