If there's nothing on the disk yet, unmount all partitions (umount /dev/vda*) and start over with the partitioning. The fdisk which comes with the standard Alpine ISO is very limited. Install a more complete partitioning tool as described on the second page I linked in my previous comment. You can also format the hard disk in the server control panel to start with a clean slate.
Well I might have found the reason https://helpcenter.netcup.com/en/wiki/server…ning-kvm-server. I'd better to format in the rescue mode then.
Thank you for your reply, but it was not really the issue of the partition tool in my case.