Having the same problem as well. All other updates worked perfectly today, except this one. Surely Microsoft should realise there is a major issue with it and provide some sort of fix to rectify for the issue. I am not an IT expert and have no intention of playing around with parts of my computer I don't understand. Like other people my PC is working perfectly. My Windows Update is also showing the Defender update did not install, but it actually has as have checked my system, but this has not cleared from Windows Update today.
Update: Just had a chat with Microsoft Support. I have been told engineers are working on the issue, but when a fix will be available is anyone's guess at the moment. In the meantime, it was suggested that I pause updates for the next 7 days to stop seeing the retry button. When pause comes to an end I will see what is happening and if there is still an issue will do this again. I know this is not an ideal solution, but don't want my PC trying to install something that won't work. It has already tried it 4 times today!