Deleted 1 of 2 partitions in Mac OS High Sierra and cannot see it any more with Disk Utility
You received the error message because the Apple_KernalCoreDump partition resides immediately after the Apple_APFS partition. This I was able to verify.

I can not think of a reason to keep the Apple_KernalCoreDump partition. So, I would enter the commands given below to add the free space to the Apple_APFS partition.

diskutil  erasevolume  free  n  disk0s3
diskutil  apfs  resizeContainer  disk0s2  0


If after deletion, the Apple_KernalCoreDump partition reappears, then I would assume you drive is corrupted. In this case, I would backup important files, repartition the drive and reinstall macOS.