What do you mean they are not allowing you to resign? Resignation does not need approval from the company. You resign. Which is an intimation in written to tell them you are out. If your contract has a notice period, you serve it and you are out on the last day.
A last ditch effort would be to just leave the internship today. I mean, stop going and stop responding. The worst they can do is not pay you for that month and not give you a certificate. Considering you are interning, I am assuming the full time job is your first job. If so, you are a fresher anyway and they aren't gonna ask you for any kind of experience letter. Don't do this if you don't need to. And read your contract for abandonment and bond criteria before even considering it. But, it's an option.
Ideally, just write an email that you are resigning. Either by mail or mail and some HR platform they might be using. But make sure you have it in written and you have screenshots. Once your notice period ends, just stop showing up.