How do you allocate money left over in one month to the next month in YNAB?

Go into the next month and allocate it there. If you over allocate a category in the current month, it will roll over, but YNAB will still prompt you to assign that month's target for that category and ignores the Available amount.

Thanks

Btw, apparently that's a feature, not a bug

I would throw in that there is 2 ways of putting away for next month, 1 what @abundant-psychiatrist-22252 mentioned, the other is to have a next month category, each one has its own pros and cons, it’s up to personnel preference.

I saw the next month idea somewhere. What's the benefit of doing it that way?

Actually with their latest changes, they don’t recommend it anymore. https://support.ynab.com/getting-a-month-ahead-HJidy13C5