Description
The value at each cell location on the cost raster represents the cost-per-unit distance for moving through the cell. Each cell location value is multiplied by the cell resolution while also compensating for diagonal movement to obtain the total cost of passing through the cell.
The values of the cost raster can be integer or floating point, but they cannot be negative or zero (you cannot have a negative or zero cost).
Solution
Update the cost raster input so that there are no values that are zero or a negative value.