In case you are using custom variables as in the ones not defined in Houdini by default (like $JOB or $HOME for example) you must define them in our GridMarkets ROP. Please also note that the environment variables that are defined in Windows or Mac OS by default won't work either as we do use Linux on the nodes - so those would have to be defined in our ROP as well.
In case the given variable is already defined in the scene or houdini.env the "Env Value" field should automatically fill up after editing "Env Name" field and pressing Tab.
In order to define the variables go to the GridMarkets ROP > Environment > use "+" to add a variable. Please note to use only forward slashes: "/". Consult the image below for an example: