You can't do "sections" by just writing stuff in the file. Then the game comes to the line that says "ufo components" and it doesn't know what that means so it stops reading and gives an error.
Anytime you write something in the file that is not in OpenXCom-yaml ruleset syntax, you will get errors. In this case, since it is just for your ease of reading the ruleset and does nothing rules-wise, this is what we call a comment. You need to precede comments with the appropriate marker, in this case a #, and then the game will know that it's not supposed to read that. Something like:
items:
# ufo components:
- type: STR_ALIEN_ALLOYS
listOrder: 1
- type: STR_ELERIUM_115
listOrder: 2
# craft weapons: