I'm not an inventory expert at all, but when fulfilling a Sales Order, why would an Item Fulfillment's Inventory Detail have a Serial/Lot that is selectable but when saving it says the quantity isn't available for the Lot and Bin? The correct Bin auto selects but then it fails to save. It seems like there's none in the Bin but there's quantity available int he Lot? On the item's inventory tab, the Bin Number tab is showing (1) or -1 but on the Inventory Numbers tab, the Lot is showing that there's 4 on hand. They did an Inventory Adjustment but it didn't seem to do anything.

Is there only one Bin? On inventory-adding transactions, bins are generally not required (unless a preference is enabled which still does not make every positive inventory transaction or a customization). To check if there are items not put in a bin, we generally go to Transactions > Inventory > Bin Put Away Worksheet and move it to from a no bin to a bin from there. Do you see the item there?

They don't use Bin Putaway Worksheets. They use Inventory Adjustments to move some things around. I believe the issue was that they created an Inventory Adjustment to adjust quantity and did not use the correct serial/lot which put the inventory somewhere else and then it wasn't available for that lot. Once I created an adjustment on the serial/lot that was having issues, I could see quantity in the bin and the Item Fulfillment saved fine.