I noticed the same thing, and tried the same. Aside from the above error message, I also noticed this in the installation log:
plugin: installing: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/plugins/preclear.disk.plg
plugin: downloading https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/plugins/preclear.disk.plg
plugin: downloading: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/plugins/preclear.disk.plg ... done
plugin: downloading: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/archive/preclear.disk-2020.05.05a.txz ... done
plugin: downloading: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/archive/preclear.disk-2020.05.05a.md5 ... done
plugin: downloading: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/source/packages/readvz ... done
plugin: downloading: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/source/packages/dialog-1.3_20200327-x86_64-1.txz ... done
plugin: downloading: https://raw.githubusercontent.com/gfjardim/unRAID-plugins/master/source/packages/preclear_disk_20.sh ... done
tmux version 3.0a is lower than the installed version (3.1.0), aborting...
libevent version 2.1.11 is greater or equal than the installed version (2.1.11), installing...
+==============================================================================
| Skipping package libevent-2.1.11-x86_64-1 (already installed)
+==============================================================================
utempter version 1.1.6 is lower than the installed version (1.1.6.20191231), aborting...
+==============================================================================
| Installing new package /boot/config/plugins/preclear.disk/preclear.disk-2020.05.05a.txz
+==============================================================================
Verifying package preclear.disk-2020.05.05a.txz.
Installing package preclear.disk-2020.05.05a.txz:
PACKAGE DESCRIPTION:
Package preclear.disk-2020.05.05a.txz installed.
-----------------------------------------------------------
preclear.disk has been installed.
Copyright 2015-2020, gfjardim
Version: 2020.05.05a
-----------------------------------------------------------
plugin: installed
Updating Support Links
preclear.disk --> http://lime-technology.com/forum/index.php?topic=39985.0
Finished Installing. If the DONE button did not appear, then you will need to click the red X in the top right corner
#
I will assume that in my case the Nerd Tools package upgraded some pre-requisites to a version higher than the script likes.