How to Purchase Sandbox
There is still another hotfix incoming for 3.21, but we hope this helps you get online and playing with Sandbox right away!
Step 1. Ensure Steam funds are added before you click purchase in-app
Clicking the 'Get Sandbox' button will open the Steam overlay.
If you do not have enough funds in your wallet to purchase, the purchase may fail!
Step 2. If you need to add funds, do so in the Steam app first
Funds can be added from the your username dropdown menu on the top right of the Steam app > View my wallet
Once you have added sufficient funds you should be able to purchase.
If this doesn't work, a purchase from mobile should
A purchase from a linked iOS/android device will also work on Steam.