Sforno on Deuteronomy 1:26/ספורנו על דברים א׳:כ״ו

1ולא אביתם לעלות, for at that point you revealed your original sinful approach, i.e. that your motivation had not been to select the best part of the country for yourselves at this time, as I had thought, based on what you had said. Your purpose in the mission had been to determine if it was possible to conquer this land, something that revealed that you did not believe G’d’s promise to you. As a result of your lack of belief you accepted what the spies told you when they said that these people are too strong, and you refused to ascend to this land.