You should accept the walk-in or at least accept the appointment of that day. I was late by 20 minutes. And I have to schedule for next year?

I think if your group for the time block is still outside waiting they will be able to let you in, but if they are all inside already, you probably will need to reschedule online. And whenever you schedule online you can see which DMV has less people as well.

If you try, I was told that some of the other DMVs take walk ins. You will just have to check around

Well I don't work there. But I do know ot says when they e-mail you that if you are late you'll have to reschedule.