If you are receiving this error when attempting to make purchases inside Hearthstone, it means that you or someone else (hopefully a parent) has activated Parental Controls on your Battle.net account at some point in the past.
If you find yourself receiving this error, you will need to place a check next to the Allow Real-Money Auction House Usage option in the Parental Controls management screen. Afterward, you can optionally remove the parental controls entirely. We are in the process of converting this RMAH option into a generic "allow spending money in-game" option, and we apologize that the current title is misleading.
Additionally, we'll look into making the 10200249 error message more on point with what's causing it to trigger. Thank you for your patience as we continue to work our way through Beta!
EDIT: If the Allow Real-Money Auction House Usage option doesn't show up, you may need to add a Diablo III Starter edition to toggle it. This is a known issue that we are working to fix as well. Thanks!