Here are a few potential issues that could cause the "ligand is not fragment" error message to appear:
- The ligand you are trying to dock may not be properly formatted or may contain errors. Make sure the ligand is a valid 3D structure file in a supported format (e.g. PDB, SDF)
-The ligand may not have been correctly prepared for docking. This can include issues such as missing or incorrect protonation states, incorrect tautomeric forms, or incorrect bond orders.
-There may be an issue with the docking parameters or settings you are using. Make sure you have selected the correct receptor and docking options, and that the parameters are appropriate for your ligand and receptor.