The module you are trying to create will cause the size of model to be so much that the workspace in which model is there will go over its specified limit, so workspace has a limit and sum of size of models making the workspace must not go over the limit, i.e it is preventing to create the module, also creating such a huge module is not feasible for performance issues as sparsity maybe there.
What you can do is create a numbered list by concatenating customer and items which are to be sold under a region with region as its parent, and use this list as a dimension in the module
All the responses you got are excellent. Yes, your available workspace is smaller than the model-to-be if you apply these dimensions.
My advice is, even if you had the space required, to not proceed with the lists as is. Unless each customer will be associated with all or the majority of the items then we shouldn't create all these intersections by dimensionalizing by both lists.
The idea of creating a combined list is a good idea, but also, look at lists and ensure that all items in the lists are valid (no inactive customer for example or duplicate items,..etc). Also, you can look into grouping the items and/or customers into hierarchies and plan/report on that level if your business case allows for that. then drill deeper when needed.
Einas "Give a Man a Fish, and You Feed Him for a Day. Teach a Man To Fish, and You Feed Him for a Lifetime"