Hey Gayla,
Sorry I did not get back to you earlier, I did look into this and our development team has resolved it. You are now able to assign serial numbers to Assembly Items.
You are correct in that you can only assign a serial number after you receive the Item. However, You must turn on serial number tracking before you receive the Item into its bin.
The correct steps are:
1. Turn on Serial Number Tracking in Method.
2. Receive the Items.
3. Go to the Bin for your Item, and assign a serial number under the "Serial Numbers in this Bin" tab section.