@helpstar, thanks for the info and my apologies for forgetting to reply. The issue is solved as you mentioned, but in an interesting way. All of the traditional methods to upgrade failed. On the third try, the error number indicated that one of installed apps are not compatible with v1909. So, deleted and reformatted the C: drive, but left the two hidden system drives untouched. When rebooted, Microsoft appeared on screen and began to install v1909 automatically. I was pleasantly surprised, but still don't know which software caused the problem. Since I don't miss it, it's not important. Otherwise all other installed apps are working well.