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Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
Misty (or any other staff member who can be of aid), it's clear from the lack of responses, the upvotes on my comment and repeated rejections that a portion of this community has no idea what you're looking for when you say "Finish the tutorial" as a quest objective.
Doing so is exceptionally easy*.
Proving it, however, is somewhat challenging.
If you've yet to play the game, MTG Arena does not display your chosen account name anywhere on its homepage/hub. The homepage is where you are sent after completing the tutorial. As such, there's no way of proving you didn't just grab a random picture of MTGArena off, say, Google Images and upload it... unless you click the "Profile" tab that displays the available avatars, your name and basic currency information at the top.
Again, this is the only screen (outside of a match) in which your name is actually displayed. You cannot see it in the tutorial, you cannot view it on any other splash page.
I thought it would be painfully obvious to whomever checks these things that a new account which fulfilled the quest criteria must have all of the following:
1. Access to the Profile tab
2. 500 gold
3. 0 gems
As such, a screenshot of the profile tab with said currency totals would indicate it is a new account that has completed the tutorial. For those who are unaware, gems are MTGA's "premium" currency and require real-world funds to acquire, while gold is the in-game currency you earn for winning matches; 500 are awarded after completing the tutorial, hence the stipulation regarding your currency totals outlined earlier.
...and so, I ask yet again: what is it you want to see in order to complete this quest? I cannot fathom it and the blurb appended to my rejection notification is rather unhelpful: "Something went wrong." Yes, something did: insofar as I can tell, your moderator hasn't played the game else they'd recognize the issue at hand.
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* I said "exceptionally easy" as you can literally skip the entire tutorial, while still receiving all the rewards that an individual who plays through it will acquire. Merely hit Escape and select "Skip the tutorial." You'll be taken to the final rewards page, then deposited onto the game's homepage/hub.