Your Target is a Player Group variable when it's supposed to be a Unit variable. Think about it, the Caster is the one attacking, and the Target is the one being attacked.
You cannot order a unit to cast a spell targeting "All allies of some player". That's telling the game to cast Ray of Disruption on Players, which is impossible. The target of the spell has to be a specific unit.
Here's how you could make the Attacking unit cast Ray on a nearby allied unit. Note that like I said before, Ray of Disruption
CANNOT be used if the target has no buffs to dispel. That's why I check to see if the picked unit has buffs.
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Ray of Disruption
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Events
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Unit - A unit Is attacked
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Conditions
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Actions
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Set VariableSet Caster = (Attacking unit)
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Set VariableSet Target = No unit
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Set VariableSet Point = (Position of Caster)
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-------- --------
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Custom script: set bj_wantDestroyGroup = true
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Unit Group - Pick every unit in (Units within 800.00 of Point.) and do (Actions)
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Loop - Actions
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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((Picked unit) belongs to an ally of (Owner of Caster).) Equal to True
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(Number of Negative buffs considered Magic or physical on (Picked unit) (Exclude expiration timers, Exclude auras)) Greater than 0
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Then - Actions
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Set VariableSet Target = (Picked unit)
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Else - Actions
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-------- --------
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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Target Not equal to No unit
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Then - Actions
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Custom script: call IssueTargetOrderById(udg_Caster, 852615, udg_Target)
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Else - Actions
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-------- --------
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Custom script: call RemoveLocation (udg_Point)
So this trigger will only work if there's an allied unit within 800 range of your Attacking unit, and only if it has a negative buff like "Slow" for example. Also, note that I left out the condition for your Void Seer unit, make sure to add that in.