You have to use the Migration Tool to go from 8.1 to 9.0 first, then you can upgrade to whatever...
The OS has to to be upgraded to upgraded to Linux going from 8 to 9 and the partitions reformatted and boot code changes completely... it's changes a ton of stuff under the hood, it's all automated but takes some time.
There is no way to go to any version above 9 from any version below 9 directly, you must go to 9 first.