What do I do if someone is blocking the way into the Elite Four.?

They are blocking he elite 4 becaus Loreili isn't there. Go into the Four Island Cave and help her defeat the Team Rocket members.

Collect 60 Pokemon to get the National Dex from Oak, go to Island Four, help Lorelie in Icefall Cave, train your Pokemon good, and prepare for a challenge at the Elite Four.

You must obtain all eight badges from the gym leaders.

I think you should go to One Island and talk to Celio. He might have something for you to do.

Return to One Island as I remember, talk to the guy who stand right next to the big machine, he will ask you to find the Ruby and Sapphire. The Ruby is on the mountain of the same island where 2 member of Team Rocket is standing. Take the Ruby back to the Pokemon Center and give it to the guy I mentioned. He will give you Rainbow pass which will allows you to travel to all 7 islands. Go to 4th island, enter the cave and go to the inner cave, solve the puzzle inside and get Waterfall, return to the main of the cave and teach waterfall to 1 of your Pokemon and use to get to the top, go down the ladder and find Lorelei, you have to battle 1 of the Rocket.

Go to Six Island, go south and you will find a ruin, use Cut on the door and jump down the hole, jump on the hole up then left then right the down. You will land beside the Sapphire but a scientist snatch it away. You have to follow him to Five Island, go to the meadow and enter the building, solve the puzzle. Then you end up fighting with some of Rocket and finally, you have to defeat the scientist to get back the Sapphire, hand it to the guy on One Island and then you are granted access to the Unknown Dungeon (Mewtwo) and the harder Elite Four