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I've watched the Nick True video that came highly recommended in a bunch of Reddit results (e.g. here), but I still don't understand what's going on.
Hmm I don't think it should. Have you checked there are no negative balances in any of your categories?
Confirmed, all categories have non-negative amounts in the "Available" column.
I'd recommend reaching out to YNAB support. They can even view your budget (if you give them permission) and determine if it is a bug or something is configured wrong.
After you have the money in Ready To Assign, did you have an overspent credit card category in the budget? If I try to replicate what you did, I do indeed have the money in Ready to Assign but I also have an overspent credit card category, and the fix would be to assign the RTA money to that credit card because you sent more to the credit card than you had set aside for the payment. If that's the issue, see https://support.ynab.com/en_us/when-your-credit-card-payment-category-is-red-a-guide-SJDSr3Q1i
That said, I usually don't have to do this so perhaps the reason is that you had some credit card payments that weren't budgeted for (maybe you only started with YNAB recently?).