Do you already have a letter ID assosciated with your letters of rec?
Interfolio->AMCAS requires that you already have a letter ID, so AMCAS can tell which letters are for who.
If you don't already have a letter ID, then you need to add the letter on AMCAS first, and then it will give you the letter ID (lets say your letter writer's name is John Doe, then just say that john doe is going to write you a letter and AMCAS will assign you a letter ID number, lets say its 15555559)
Then, go to interfolio and send the letter from John Doe and it will ask you what the AMCAS letter ID is before you even send the letter. Put 1555555559 for it, so when amcas recieves the letter it knows it's a letter for YOU from JOHN DOE.