JhonBondiola Do this first
Close BO2 completely.
Close CB Launcher too.
Press Win + R.
Paste:
%localappdata%\Plutonium\storage\t6\players
Find:
plutonium_mp.cfg
Move it to your desktop rather than deleting it.
Reopen CB Launcher → launch BO2.
Have your friend do the same thing on their PC.
Both of you restart BO2 and try the private Custom Game again.
Plutonium staff specifically recommend deleting/resetting the plutonium_mp.cfg config when T6 settings become corrupted; the game will recreate it with defaults.