“ And, behold, this day I am going the way of all the earth, and you know in all your hearts, and in all your souls, that not one thing has failed of all the good things which YHWH your God has spoken concerning you. All have come about for you. Not one thing has failed of them.”
This was not to say that he was about to die that day, but that he was aware of his age and aware that it could not long be delayed. Like all men he must die. (‘This day' connects with ‘behold').
He called them to admit that in their heart of hearts they knew that God had been faithful to them and had fulfilled His promises. Here they were, established in a good land and with a good future before them as long as they remained true to Him.