Put on your solution caps please! So I've drawn half a shape (leaving the second half incomplete) in order to copy it, mirror it round, and marry both open sides together to give me a complete unified symmetrical shape. Nothing too fancy to do right? Well try telling my computer that!
I find I do this maneuver many times, but from time to time I'll encounter this mind boggler of a problem! Now that I'm finally a member here I feel I finally have an opportune place to bring this issue to light, and murder the inconvenience it always imposes!!!

THE RUNDOWN:
• So I bring both open sides of the 2 shapes together.
• Then I select the anchor points I'm wanting to join, and go to "Object>Path>Join" (⌘+J).
• Usually at this point it'll join and I'll proceed to join the last pair of anchor points.
• But instead, given my current situation, a message keeps popping up:
To join, you must select two open endpoints. If they are not on the same path, they cannot be on text paths nor inside graphs, and if both of them are grouped, they must be in the same group.
... and neither of the shapes are grouped, or on a text path.
Really stuck here! Please help.