Ah, here’s the issue with that. The prince or the king must approve of the divorce for it to be valid or allowed. Women during this era could only ask their husbands for a divorce but if the husband did not agree they could not receive it—however the men were allowed to divorce their wives without consent. The only way to get around that would be to beg the king to annul the marriage or allow the divorce but 1: she would never have been granted an audience because it was clear the prince would have continued to lock her up, and 2: the king happens to be very sick and bedridden so that’s impossible currently.
Iirc it was because the emperor kept pushing it back to try and make FL stay longer. Like if she wanted to marry tomorrow he'd say to wait. So there was never a guarantee she'd have the ring even after they got married because he knew that was all she wanted
Did I miss this being said in the webtoon?
I'm so pissed cus y u gon escape 2 days before ur wedding (which literally solves 80% of your problem btw). Like just get married, take the ring, get a divorce then run away. PATIENCE AND A LIL BIT OF WIT COULDVE HELPED U ESCAPE GRL.