Wall of Roots

• WOTC Official Reminder Textofficial rules
  Defender — This creature can't attack.

• WWizards of the Coast2017-11-17
  Official Ruling
  If you must sacrifice a creature to pay a casting or activation cost that also includes mana, such as that of Bubbling Cauldron's abilities, you may put a -0/-1 counter on Wall of Roots to make its toughness 0 and then sacrifice it to pay that cost.

Rulings via Scryfall