The correct version is “something to that effect.” In this instance, “effect” is part of an expression and can’t be replaced without changing the meaning of the expression.
However, in other instances, you can determine whether you should use affect or effect by replacing “effect” with a similar noun (such as “outcome”).
If the sentence still makes sense, “effect” is correct.
If it doesn’t, you probably mean “affect.”
When in doubt, use QuillBot’s free grammar checker to prevent mistakes!