Require support to sort out Atom Type Error
A protein PDB_ID: 1US0 was docked with Shikonin molecule using GOLD. 
The docked complex was subjected to MD Simulation using Biovia Discovery Studio.
Before running simulation, Force field was applied.
Under the section “All Forcefield”, we selected charmm36 under Forcefield.
It reported error when we clicked Applied Forcefield.
On clicking More, the error described “Atom A:LEU17:?5944 has an unknown element type, which cannot be typed by the selected forcefield.”  
Please guide in this regard to proceed further as other options related to Force field customization remain inactive even after selecting the molecule.