Out of diapers!!!
I counted down for a week with both my kids to the last diaper .. And told them they don't make them for their size and its time.. Obviously we had been practicing but then that last diaper was go time.
We had some accidents in the first two days. A lot while out.. But I was prepared with clothes and patience.
After two accidents they got it. They didn't want to have an accident ..they learned quickly and we were fully potty trained under a week.
It's the parents. You gotta stick to it, be tough but warm and kind.