Wall of Roots cannot be activated during mana payment steps, only at instant speed.
Wall of Roots cannot be activated during mana payment steps, only at instant speed.
Have Wall of Roots, Raging Goblin, and Giant Growth in hand
Cast Wall of Roots and Raging Goblin
Go to combat and attack with Raging Gobin
Left-click Giant Growth and select the Cast action
Actual: Cannot activate Wall of Roots to pay for Giant Growth
Expected: Can activate Wall of Roots ability to put a -0/-1 counter on it to pay for Giant Growth
Notes: You can use Wall of Roots to add mana to your mana pool before trying to cast a spell to mitigate this issue. There are still scenarios where this won’t work, such as:
Have Wall of Roots and Chord of Calling in hand
Cast Wall of Roots.
Activate Wall of Roots four times
Left-click Chord of Calling and select the cast action
Actual: Cannot tap Wall of Roots for convoke cost and use its ability to pay two mana for Chord of Calling’s cost
Expected: Can tap Wall of Roots for convoke and then activate its ability to pay Chord of Calling’s cost
This issue also affects:
• Kozilek’s Translator
• Chromatic Sphere
• Selvala, Explorer Returned
• Mana Bloom
Due to the complexity of the issue and how it relates to the underlying MTGO game infrastructure there is no ETA for a fix.

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Luckyfinish commented
a statement, why it isnt fixed yet, would be nice
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hmj321#05508 commented
July 2021 and the issue persists.... I still can not use wall of roots when convoking chord of calling and removing the last counter.
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Masumaro#06926 commented
besides convoke, you also can not pay for cards like ghostly prison, also if you are nor aware of the bug you might not be able to pay for manaleak