LINGO's SP solver generates a deterministic model to solve the original SP model.  This model is called the deterministic equivalent (deteq).  Depending on the number of stages, random variables and samples sizes, the deteq may get to be very large.  In which case, there is a possibility that it will exceed the capacity of your LINGO license.  This can occur even when the core model lies well within the capacity of your license.  You will need to either reduce the size of the model (by using fewer stages, less random variables or smaller sample sizes), or upgrade to a license with more capacity.