Meeting room permissions were recently reset so you can't enter meetings directly into those calendar anymore.
The process should be to send a meeting invite to the room & attendess, the room will automatically accept if it is free
Either click New Meeting from the toolbar or right click in your own or a fee earners calendar to arrange on their behalf > New meeting Request
Enter the meeting information as normal including the Location which is the meeting room of your choice. Once you send the request you will receive a notification if the room has accepted/denied your request:
