Item identification cannot be changed when inventory transactions have been generated.

I'm trying to insert sales line for couple of items and I run into this error always...not sure how to get rid of the Inventmovement class issue.

I tried creating a new InventDim Id by 

 inventDim.clear();

//                inventDim.InventSiteId = inventSiteId;

//                inventDim.InventLocationId = inventSiteId;

//

                salesLine.InventDimId = InventDim::findOrCreate(inventDim).inventDimId;

Can you please help what needs to be done here to proceed? 

Thanks,

Ken