You probably need to make sure the cursor is on the right element and its color is green. An easier way is to use the "Move" button at the top of the page. Select the item you want to move then you will be presented with a dialogue like this that allows you to place the item exactly where you want
If that doesn't get you the result you are expecting, send me a screenshot of you are trying to accomplish.
Einas "Give a Man a Fish, and You Feed Him for a Day. Teach a Man To Fish, and You Feed Him for a Lifetime"
If Product C has to be the amalgamation of D & E, you have to set Product C as Parent of D & E. You can not move list item into another list item of same level and merge them. Please review short tutorials shared by @lindascott
If you want parent-child relationships, it must be 2 lists, and they will be related in a hierarchy. That's the essence of a composite list. Your lists might be:
P1 Product (these are the parents) - Product A, Product B, etc.
P2 Sub-product (these are the children) - Product b1, Product b2, etc.
Each parent does not need to have children, though if you want to be able to compare Product A to Product b1, you would want to structure Product A as a single child a1 and then roll it up to A.
If you have not taken the full Level 1 Model Building course available on demand in the Anaplan Academy Learning Center, I highly recommend it. You will get thorough instruction in the use of lists and how they work. The micro-lessons that I posted the links for earlier are taken from that course, but you'll get a lot more context for the content along with an opportunity to build a practice model.