macOS Mojave Security Update 2021-004 10.14.6

There are 3 early steps you should run when facing any problems that could help solving thing without big efforts. They usually help a lot.

Reset SMC How to reset the SMC of your Mac - Apple Support

Reset NVRAM Reset NVRAM or PRAM on your Mac - Apple Support

Restart holding Shift to enter Safe Mode, wait it to load (takes a bit longer), restart normally and try updating again.

Come back if not OK so we can look deeper.

iZeZo wrote:

I couldn't install macOS Mojave Security Update 2021-004 10.14.6 . It fails with ERROR_7E7AEE96CA displayed in dialog with Catalina install badge. How to fix that?

I recall ready somewhere that this error can result from having a system that was on Catalina and was incorrectly or incompletely "downgraded" back to Mojave. It can be caused by leftover Catalina files or operating system elements that interfere. Did your Mac have Catalina at some point running? The fact that a Catalina icon appears is further indicative of this. Do you have Catalina previously installed, or is this a second hand Mac?

If you did have Catalina before, and the steps suggested by Rafael do not help, if I were in this situation I think I would make sure all key files were backed up and then erase/format the drive and then with Migrate Assistant migrate over only files/folder, no settings and no apps. Then reinstall all non-Apple apps from scratch. You must have reliable backups for this as you would be erasing the disk.

Unfortunately, it looks like your reverting to Mojave from Catalina has resulted in a flawed OS, so the Security Update can't be applied. It seems that maybe the only consequence of this is not having that little extra protection from that Security Update (and probably all subsequent ones)? You could choose to simply stay with what you have, or if you are not satisfied with that, make some good backups of all your files and then erase/reinstall Mojave from scratch and then migrate over only files (no settings nor applications).

With this glitch I'm also not sure whether you can smoothly transition to Catalina or Big Sur or follow on OS versions in the future without having this addressed. If you expect to stay with Mojave forever on this Mac, maybe you could choose to just accept it as it is.