ECode12

I've been getting this same error since I left the Abandoned Apartment. I needed Blast Powder so I left the building and now I get this message every time I step into the grassy patch.

I'm having trouble finding a couple pokes and would immensely appreciate some help! Mainly, I can't seem to find Shuckle in the listed locations, and are Plusle and Minun static encounters or just super rare to find in the wild? I also wanted to ask, is there a way to find Cacnea and Beldum? I've been seeing some mentions of a "third fragment" in Darchlight cave but I have no idea what that's referring to. Thanks again!