I reached level 43 where you need to research a Legendary combo. I prepared for this so 2 days ago, I researched the event card Bitch Girl 3 times and waited until level 43 before I would click the ready button. After completing the research and clicking ready, it did not registered that I did the research and I can't get passed level 43.

My Game ID is 7490909.

Hello!  This is not a legendary combo, the blue color of the card indicates that. Legendary cards are red.

A legendary card and an common card can never turn into a legendary combo

As @Hooligart states, the color of the combo is relevant - not the color of the cards involved.

A "legendary combo" is one that results in a legendary (red), not one that involves a legendary character, or legendary item.

A legendary (red) character + a common (grey) item makes an epic (blue) combo.