If you're seeing this box appear when attempting to launch the game in Steam, follow this guide to fix the issue.
1. Please try re-installing or verifying the game files. Once it has finished installing, restart Steam by quitting the Steam client here. Then just open up Steam again.
2. Right click Rec Room on your Steam Library and click the Preferences option
3. Click Set Launch Options
4. Make sure that the text box is clear of any text and click "OK"