A hotfix has gone out to fix the matchmaking issue due to golden cards and Secrets not being discoverable from a couple of cards.

Quote From cmdylux

Hey! Going to hijack the sticky here. We just released a hotfix addressing the matchmaking error for Standard, Wild, Duels, and Casual. It might take a little while for this to deploy across all regions, but once it has you should be able to queue for these modes without issue. Also, a fix for Secrets from the Core Set not being Discoverable from Ringling's Rifle and Ring Toss went out at the same time.