I prefer the ones where you just enter the registration number and be done with it. Is there anyway I can fix this.
This is exactly like what you are describing, you just enter the registration and be done with it. Fact there's an activation is no different than any other software, including FSX and Windows itself...
It's just normal you would get an activation failure message if you updated your system, it should be enough to simply select "Register Serial" and insert your serial to reactivate it.
If, instead, you got a generic "Esellerate failure" message without any code, it means there's a problem with your user account permission, running Flight sim As Administrator, at least once, should fix the issue.
Of course, it would have been helpful if you said which Flight sim you use and under which OS