Thermos-1.7.10-1614-server

Plugins list:

• Essentials
• EssentialsChat
• LuckPerms-Bukkit-Legacy-2.17.39
• Vault-1.4.1.jar

admin.yml

name: admin
perms: {}

Command:
/lp group admin permission set test.test

Server response:
Set test.test to true for admin

Command
/lp group admin permission set test.testt

Server response:
Group could not be fond.

admin.yml

name: admin
perms:
  test.test: null

Server log please.

https://gist.github.com/Westus/e610092d1e797b81f65521ed750d3f89

Please delete the admin.yml file & try again. :)

https://gist.github.com/Westus/e71fd47bda96b271b7563dfd6b678302

Cannot reproduce locally.

What does the file look like once you've tried adding something to it?

Edit: just re-read your initial post

name: admin
perms:
  test.test: null

test.test should not be set to null.

Try my assembly. It already has a fixed CyberdyneCC/Thermos#498 (comment). From the first time it starts. link

Try again. For some reason, it can not run the first time.

Your server software is packaging a very old version of SnakeYAML. (a 6 year old version, to be exact)

So, in the same manner that you have to update SpecialSource, you need to update SnakeYAML.

Navigate to /<server root>/libraries/org/yaml/snakeyaml/1.9
Replace snakeyaml-1.9.jar with the file found here.
https://repo1.maven.org/maven2/org/yaml/snakeyaml/1.17/snakeyaml-1.17.jar

Obviously you'll need to rename it in the same manner as you did for SpecialSource.