lambdas can only contain expressions – basically, something that can appear on the right-hand side of an assignment statement. pass is not an expression – it doesn’t evaluate to a value, and a = pass is never legal.
Another way of thinking about it is, because lambdas implicitly return the result of their body, lambda: pass is actually equivalent to:
def f():
return pass
Which doesn’t make sense. If you really do need a no-op lambda for some reason, do lambda: None instead.