I've gotten this a few times, I screenshot my board when it happened, Did you happen to have Chromatic Lantern or something that allows lands to tap for different mana on the board? If not, then it's casting green creatures using The Myriad Pools because in the screenshots I took, I got the error when casting a green creature with it. Multiple times, and you said you got it when casting Nissa. So I'm thinking its the green creatures with it, but thinking about it now, It didn't happen when I copied Atraxa (Yes, it was a misplay) so maybe its just mono-green permanents?
I've gotten this a few times, I screenshot my board when it happened, Did you happen to have Chromatic Lantern or something that allows lands to tap for different mana on the board? If not, then it's casting green creatures using The Myriad Pools because in the screenshots I took, I got the error when casting a green creature with it. Multiple times, and you said you got it when casting Nissa. So I'm thinking its the green creatures with it, but thinking about it now, It didn't happen when I copied Atraxa (Yes, it was a misplay) so maybe its just mono-green permanents?