Minimal repro:
game:GetService("PhysicsService"):SetPartCollisionGroup(nil, "Default")
-- Error: Parameter 2 must be BasePart in SetPartCollisionGroup.
It’s supposed to be parameter 1.
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If you’re wondering why it’s printing this, in lua the colon is flavor speak for calling a function with self as the first argument.
For example, you can also call this method with the following code:
PhysicsService.SetPartCollisionGroup(PhysicsService, nil, "Default") --note the '.' instead of ':'
In other words, you’ve been calling all of your methods with an invisible argument this entire time when using a colon 
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