Troubles entering items to inventory

Good morning 

I am going to uppercut my computer! 

I currently have 16 items in stock and I need to increase to 26, everytime I go to do it I get an error message saying; 

'The value of your inventory may not be less than zero' 

I've done the trouble shooting - tried to do an inventory adjustment with no luck.

The unit cost when I go to do the 'Adjust Inventory' states  -$11.053749  (negative)

When I go into the inventory item itself - it says Average Cost is N/A and current value is N/A 

I'm so confused as to what is going on. 

HELP!