Make sure receptor and ligand have different chain IDs! You can open the pdb file with a text editor to check (column 22 of the ATOM or HETATM records https://www.wwpdb.org/documentation/file-format ).
Some Docking programs return receptor and ligand as different models/states in the PDB file, separated by MODEL/ENDMDL records in the pdb file. In this case you might need to set the display program to show all states
In Discovery Studio, after loading your docked protein-ligand complex, if it says no ligand molecules were found, you can solve this by following these steps:
In the Biovia interface, enable 'View Interactions' panel.
Select your ligand molecule.
Click on 'Define Ligand' in the 'View Interactions' panel.
By doing this, you should be able to view the interactions between your protein and ligand.