Licence number of Modeller (https://salilab.org/modeller/)
Open a PDB file by chimera.
Tools -> Model/Refine loops
Select a chain that has a missing loop of interest
OK, and then two windows appear for the sequence and modelling setting.
Select a missing residues, which may be surrounded by a red rectangle if the PDB contains the information of where the missing residues are.
Applyin the windown
Model Loops/Refine Structure.