bpy.context.view_layer.objects.active = bpy.data.collections['Collection'].objects[f"{x.name}"]
bpy.ops.object.modifier_add(type = "DECIMATE")
bpy.context.object.modifier["Decimate"].ratio = Decimate_Ratio
Decimate_Ratio is just a math equation
I've tested the code alone without putting it in a function, but now I'm required to apply it to an addon and put it in a function and all the sudden the error appears.
Except for the 's' that is missing in your code, you should avoid using bpy.ops for that.
Use for example something like this instead:
import bpy
obj = bpy.data.collections['Collection'].objects['Cube']
decimate = obj.new(name='Decimate', type='DECIMATE')
decimate.ratio = 0.5
The reasons are: bpy.ops is slower (it will cause GUI updates, etc.) and most importantly you can't have a guaranty to access the modifier by its name like bpy.context.object.modifiers["Decimate"] as Blender will name it differently in case of name conflicts (Decimate, Decimate.001, Decimate.002, ...). ObjectModifiers.new() on the other hand, adds new modifier and returns a reference to the newly added modifier:
>>> C.object.modifiers.new('Decimate', 'DECIMATE')
bpy.data.objects['Cube'].modifiers["Decimate"]
>>> C.object.modifiers.new('Decimate', 'DECIMATE')
bpy.data.objects['Cube'].modifiers["Decimate.001"]
...
