I don’t know the cause of this either, but it has been going on for months; I tried the download late last year and it did the same thing to me. There was a big discussion then from several experts on line about ways to get around the problem, but none of them seemed to work. I don’t want to go to 64-bit Catalina, and seemingly Apple doesn’t want anyone to go to Mojave, so I’m running High Sierra. Too bad, as Numbers and Pages got an update that doesn’t work below Mojave.