I think "memoir" does not support the "setspace" package. That's why you get the error when you use the environment "spacing."
If you want to change the spacing in documentclass "memoir", you can use the "setSpacing" environment.
Here is an example to show how to use it. You can set the value of "v" to any number, e.g., 0.5, 1,1.25, 5,..., depending on the spacing you want. Also, you do not need to declare any special package for setSpacing.