I am a US passenger and there was indeed a box to tick in the lower left corner of my declaration when I completed it yesterday - it reads: "Check this box to complete your health declaration, and to confirm that all guests aged five years and over on your booking will undertake a covid-19 test in line with the testing requirements for this voyage." You've got to scroll down to find the box, but there is no excuse for the system not telling you you haven't finished the form! I'll try to attach a screen shot here, edited to exclude our personal info.