Updating to Monterey 12.6 doesn't work

I've tried to install the 12.6 update 5 times on my iMac (Retina 5K, 27-inch, Late 2015 with an i5 3.3 GHz 2TB drive with .9 TB available) by clicking "update now", including from safe mode, however none of the installs have worked. The process seems to follow the usual process, but when it's all done, I'm still on 12.5.1 and I get the message that Monterey 12.6 is available and will be installed tonight. The update doesn't install overnight either. Otherwise, the iMac works fine.

How do I get the update to install?