Check your Steam Overlay settings first. It should be enabled for you to make purchases:
- Open your Steam Client;
- Go to 'Settings';
- Select 'In-Game';
- Mark with the tick 'Enable the Steam Overlay while in-game;
- Restart the game and try again.
This will enable Steam Overlay in all of your games but you can also turn it on/off in a specific game as well:
- Find War Robots in your Library → Manage menu → Properties;
- Go to the 'General' tab and find an 'Enable the Steam Overlay while in-game' checkbox.
If you're still unable to make purchases, see these troubleshooting steps by Steam here: https://support.steampowered.com/kb_article.php?ref=4453-WGHZ-7934#purfail
If nothing helps, please take a screenshot of the error you're seeing and attach it to the ticket. Don't forget to add your game ID and describe what exactly happens and how long.