I have found a temporary solution, if you put it in "limited" it lets you save the changes and when you put it in 3v3 you can play normally

I just spent 50 dollars on this game to build a deck and I can’t even test it properly.. just wanna know how long this will take to resolve?

Had this happen as well: Steps to repro:

1. Enter Deck building
2. Create deck
3. In deck options - choose "Standard" under format
4. Add 8 cards to the sideboard
5. Note the error - "Cannot have more than 15 cards in a sideboard."

As someone who loves to deck build and frequently brews in arena: This is really obnoxious. I changed it to "direct match" deck to get around it as "traditional Standard" is no longer an option.

same here

I had a similar issue.

My fix was to go to the deckbuilder, click on the "more details" deck image, and adjust the deck format to "Traditional _____" (standard, in your case). Afterwards, the deck saved fine.