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.