Question:

What does the following error mean:

Last update on 2021-05-01 11:34:02 failed with the following error:
Appliance localhost returns error code 1: Error in PREIN scriptlet in rpm package nxengine-6.30.2.0-1.x86_64
nxengine-6.29.1.0-1.x86_64 was supposed to be removed but is not!

Answer:

Error means that for some reason during the update process older package was not removed and therefore a new package not installed.

Most common root cause is insufficient free disk space on the Appliance where error message happen.
You can verify free disk space in the WebConsole of the Engine in General section:

as well as running command via ssh/cli: 

df -h
CODE

You should have at least 10 Gb free space for the update.

Once the disk space is increased you can proceed to attempt another update.

If the disk space is not an issue, we recommend checking the hardware requirements and specially connectivity requirements for the specific version you will update to.