No single types or double types create a Pokemon with no weakness. If this happened, some Pokemon would be overpowered than others which can result into a Pokemon with an OU tiers - seeing that other Pokemon with many weaknesses already scored a spot in the OU tier.
If you want a Pokemon with no weakness, catch and raise an "Eelektross". This Pokemon is a single electric type with the ability "Levitate". Therefore, he has no weaknesses. Other single electric type Pokemon with an "Air Balloon" can substitute it, but can't provide the same effect - since it can wear off. As a result, to prevent Eelektross from being overpowered, his stats were lowered to now being not yet the strongest or weakest electric type.
Hope I helped! :)