"Our door is always open."
Translation:Naše dveře jsou vždy otevřené.
Translations using "vždycky" are (and have been) accepted. Perhaps there was something else in your answer that Duo didn’t like. For troubleshooting, it’s best to either (1) use the report button in addition to commenting, or (2) include a screenshot in your comment, so that the team can see your complete answer. (Reporting is easier.)