The error message means that Graphab does not have enough memory to perform the processing.
You must increase the memory available for Graphab in the menu File -> Preferences -> Memory. You can increase it up to the limit of your computer's RAM minus 1 GB.
After having modified it, re-run Graphab and check if the memory in the Graphab preferences has been modified.
If you can not increase memory more than 1 or 2 GB while your computer has 4 or more, you must have a 32-bit version of Java. Uninstall all installed Java on your machine and install Java 64-bit:
When creating a project in Graphab, the landuse map must be in integer format ie. each pixel stores an integer. Decimal formats (Float32 and Float64) are not supported. If you encounter this error, convert your raster to an integer type using QGis or any other GIS before you create your Graphab project.
In Graphab the cumulative costs are not multiplied by the resolution (pixel size in meters), so you may very well have a cumulative cost lower than the metric length. It all depends on your resolution.