I've just tested it and it seems to work fine for me. Are you sure you just don't have another plugin affecting weather that is overwriting this plugin?
Okay, I've tested every scenario I can think of. Here are my results:
When the ability Snow Warning activates OR when the Player uses the move Snowscape:
Animation plays properly and the "Snow started to fall!" message displays.
In between turns (while snow is active):
Animation plays, but no text appears.
When the AI chooses the move Snowscape:
Before any Pokemon perform their moves, this error message appears:
=================
[2023-06-30 22:09:58 -0700]
[Pokémon Essentials version 20.1]
[v20.1 Hotfixes 1.0.7]
Exception: NameError
Message: uninitialized constant Settings::HAIL_MODE
Backtrace:
[Improved AI] AI_EffectScores.rb:110:in `pbGetMoveScoreFunctionCode'
[Consistent AI] AI Move EffectScores.rb:19:in `pbGetMoveScoreFunctionCode'
[Consistent AI] AI Move.rb:119:in `pbGetMoveScore'
AI_Move:127:in `pbRegisterMoveTrainer'
[Consistent AI] AI Move.rb:22:in `block in pbChooseMoves'
Battle_Battler:455:in `block in eachMoveWithIndex'
Battle_Battler:455:in `each'
Battle_Battler:455:in `each_with_index'
Battle_Battler:455:in `eachMoveWithIndex'
[Consistent AI] AI Move.rb:17:in `pbChooseMoves'
The game does not crash and the battle continues. Other Pokemon are able to perform their action but the Pokemon attempting to use snowscape has their turn skipped. This seems to be an issue with the Improved AI plugin. They try to call HAIL_MODE which I can only assume was used in an older version of the Gen 9 Pack but is no longer used.
When entering a battle with weather snow active in the overworld (as defined in map metadata):
The message "Hail is falling" appears at the start of the battle. This should correspond to line 296 in Battle_StartAndEnd of the default Essentials scripts, since editing that line of text fixed the issue.
In between turns: Same as before, where the animation plays but no text appears. My Meowscarada did not take damage suggesting that Snow was called instead of Hail, and that it is just a text issue.