The response message:
You are trying to check out a guest from a door which is already vacant or you are trying to add a companion guest into an already vacant room.
Why this happens:
This can occur especially if you have a hostel setting in the Data JSON field for the key encoder configured in your key cutter integration. When you have included the line "DuplicateKeyAllowed": "True" in the Data JSON, Mews always creates a joiner/duplicate card. When there are no valid cards registered in your door lock software linked to the space for which you are trying to create a key, the door lock software will throw this error.
How to resolve:
Check if any of your key encoders configured in the key cutter integration in Mews uses the hostel setting.
Delete any previously created key in that space,
And then try to create a new key for that space, directly in your door lock software
Then try to create a key from Mews, and observe whether the same error message occurs (see How to find logs from the Mews Open API app)
If your property is not a hostel, you do not need to have the line
"DuplicateKeyAllowed": "True" in your key encoder configuration.
If your property is a hostel, but you have individual bed spaces configured in your door lock software, and you have mapped these spaces in Mews (Key cutter integration > Key cutter mappings), then you do not need the line