Creating a validation rule in SF at the Opportunity level

Hello everyone, I need some help, and hoping some of you can lead a kind hand. 

I am very new to creating validation rules in SF, I need to create a validation rule in SF that, when an opportunity in sf is completed (a completion date is entered), the staff is obligated to enter the expected impacts (created as a Picklist multi select)...and I cant seem to get the formula right, mostly because I dont completely understand the functions i need to use to get it right....Please help, and thank you in advance!

Mila