The response message:
You cannot create any more cards for this room. In the lock software, create a brand-new key which will delete all of the previous keys. Read more here:
Why this happens:
In some door lock softwares, there may be a limit of how many valid cards there can be registered in the system for one particular room or door. When this amount has been reached in the system, the door lock software will not allow any more cards to be created, either directly in your door lock software or from Mews.
How to resolve:
Create a brand-new key in your lock software, which will invalidate all previous keys, and you can then try to create keys from Mews.