The messages at the bottom of MO2 indicate that MO2 is running properly - they just say that it spawned the process (FNIS) and waited for it to complete. In MO2's eyes, that means that the process shut down, which it did. That said, the problem MUST be in FNIS, and for that, you might have better luck asking in the Nexus Forums than here... I've never had problems with the program, and assuming that the GamerPoet video you used is up to date and that you followed all listed instructions, this is more likely something that the author can help with than us. Especially since you verified you aren't (and have never) using ModDrop.
Thinking on it, I do have one question - which version of Skyrim are we talking about here (LE or SE) and are you using the matching version of FNIS?
Well I already ask on the FNIS forum on nexus. The author say me something MAYBE related to a problem about FNIS running but the window is hidden by MO2. He don't remember the solution, and I didn't find the exact point on the nexus forum. After all, the author tell me to ask MO2 authors because for him it is mo2 related......So now I am on the MO2 forum (my post here https://github.com/M...izer/issues/573), and have no solution yet. I just became crazy with that.
My skyrim is the Special edition very last version ( 1.5.53.08) and FNIS Behavior 7_4_5. I can play skyrim now, but i just don't get few mods I would like.....