Sometimes a record/case/appointment may be locked by another user, this could be due to a number of reasons; they are logged in and editing the record/case which is why this would stop you, another reason could be they forgot to log out when leaving, their computer shut down with no reason. Below is an article on how to clear these locks for any time you may come across this issue.
When performing an action on TM3, you may receive this pop up explaining your action is already locked by another user.
You can clear these locks by accessing your 'Settings > Configuration' which you will then be provided with the screen below. Clear Locks will be the last option within the list - this is highlighted in a red box in the image below.
PLEASE NOTE; Before clearing the locks please ensure that the user who is stated on the message has finished with the record, as removing the lock will allow you to access the record which may result in data being overwritten if they are working on the record/case.
When you click on 'Clear' you will be given a confirmation like the image below. If you are sure you want to clear the locks then click 'Yes' otherwise click 'No' if you aren't sure on the process.