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Have you disputed anything else online with TU within the last 30 days?
Hi and welcome @Anonymous
It's best if disputes are done via certified mail. Make sure you attach all supporting evidence and be very clear in your letter what you're disputing.
If online dispute process is what you want to do, you may try doing it through Credit Karma's website. I'd recommend against it, but it's certainly an option
It may just be a browser issue. Switch to another and try again. Is this the page you started on?
https://www.transunion.com/credit-disputes/dispute-your-credit