FMV stands for fair market value. It can't be zero, which must be why you're getting that error message.
If you're trying to depreciate an asset, go to the Schedule C or E and change the asset that you're depreciating to have a value, usually the price you paid for it at the time of purchase.
For more information on how to figure FMV, see here, especially this part:
"
Fair market value (FMV).
FMV is the price at which property would change hands between a buyer and a seller, neither having to buy or sell, and both having reasonable knowledge of all necessary facts. Sales of similar property on or about the same date may be helpful in figuring the property's FMV."
