unable to cast instant spell
Unable to cast spell during combat - had mana available and the opponent did not have any way of stopping or preventing the spell.
This cost me my tournament.
For this issue, please submit at: https://mtgarena-support.wizards.com/hc/en-us/requests/new and select 'Report a Problem in your Event'.
Same in ranked match (see the video)
This issue is still occurring in 2023. Had one Rotpriest down with an untapped Thurnwood Falls. I was unable to cast an instant, resulting in me losing my creature and eventually the game.
In draft, on my turn got an instant spell Season of Renewal.
Turn was forwarded right into the combat phase and then to the end of turn as if I did not have spells to cast. This instant was inactive for casting the whole turn and also during opponent's turn. I had 8 or 9 mana, all unused. There were no effects on battlefield preventing me from casting.
Was playing in a Forgotten Realms draft game (first one), when I had Unexpected Windfall. I could not cast it despite have the required mana available. Either on my opponents turn, or mine. There were no conditions in which I could see that would prevent my casting the card. I even produced the required type of mana to my pool and it still would not cast.
I too had exactly this, tried to counter Saw It Coming with my own foretold Saw It Coming and the game wouldn't have it. Even when I let that go i still could not cast my counter on their Memory Deluge despite there being no reason to prevent it.
Cost me my game.