Thank you. Both of these suggestions worked- I no longer get that error message.
However, now there's a new problem- after the saving the edited file and opening it on MyBoy, it functions exactly how it did before the overwrite. There are none of the new Pokemon I added, none of the edited Natures and IVs, etc...
When I re- open the edited file on PokeHex, it also appears exactly as it did pre-edit.
I included a quick example video, where I attempted to throw a couple hacked Deoxys in my party, and the edit doesn't properly save (this is just one of many scenarios- even if I clone a legitimate Pokemon, it doesn't save). How can this problem be solved?
Edit: So I see it's trying to save the file to my computer. But even when I copy the file to the right location under MyBoy, it doesn't appear on my phone. Renaming it also doesn't seem to work.