It’s a bug; we’re aware and working on a fix. As others have pointed out, this is a warning intended for [[Mastermind’s Acquisition]] or [[Karn, the Great Creator]].
Also, though it’s worded a little strongly, it doesn’t actually require you to have a sideboard. Hitting “Ok” on the screen shown saves the deck as normal.
Wizards Jay
It’s a bug; we’re aware and working on a fix. As others have pointed out, this is a warning intended for [[Mastermind’s Acquisition]] or [[Karn, the Great Creator]].
Also, though it’s worded a little strongly, it doesn’t actually require you to have a sideboard. Hitting “Ok” on the screen shown saves the deck as normal.