When the users booking a meeting with the resource mailbox they may get below error sometime:

Error :

“The operation failed. An object could not be found. Cannot directly book a resource for this meeting”

To fix this:

Go to properties of the Resource Mailbox Calendar –> Click Permissions tab –> now you can see default Permission level set to Free/Busy –> You have to change it to Reviewer as shown in the below picture –> Apply –>Ok

Now users can successfully book the meetings with Resource Mailbox.