This error means you’re desynchronizing your thread somewhere either with task.desynchronize or RBXScriptSignal:ConnectParallel, both of which are products of Parallel Lua Beta and try to index BasePart.Touched event with it from a desynchronized thread.
While functions like RBXScriptSignal:Connect, task.delay/spawn/defer, coroutine.create/wrap create separate threads, they are not actually desynchronized threads so those functions are not the problem here.