Hello,
I have encountered this Error before and can be solved relatively easily. Essentially the Grid Field ID is the Machine Name for the grid. Each field and indeed grid has a machine name and can be viewed when you access the field in the Object Setup Page. Its is essentially the name without spaces or capital letters.
A grid is considered a field. It need to be defined on the Object Setup page. Just as there is Long Text and Single Select fields there is also Grid. Grid is essentially a table that you can display and interact with on the layout. It is essential to assign the Grid under field settings to the "Type" that you want to use the grid. When you initialise the field in point 2 essentially you are defining the field in the Context data. This is where all the information regarding inputs, outputs and content from automatons is stored. As you can see the field is an empty List [] this is where the data will be "Set" by the set grid task. It is mandatory to initialise the Grid as well as have the grid in place on one of your tabs in Layout in order to store information there. For reference sake look at how Dbot Similar Incidents is stored in a grid and this will help you to understand the flow.
Sorry for the long winded answer. Cortex XSOAR is cutting edge an a single line explanation won't do this feature justice.
PCSAE
