When I try to create an alarm template the popup message appears

You must enable a template calendar to save the template in...

What is a template calendar? How can one be created?

This may be related to a message that I get on login saying that akonadi has crashed or something similar.

To check the status of your template calendar, show the Calendars pane on the left of KAlarm's main window (use View -> Show Calendars if it isn't visible). Then in the dropdown at the top of the pane, select Alarm Templates. You should see a calendar listed there (normally titled "Alarm Templates"). Ensure that it is checked. Right click on it and select Show Details to see what its status is. To change its status, right click on it and choose Edit.

Please report back what its status was - it should be automatically enabled.

It's very unlikely that this has anything to do with Akonadi crashing, since KAlarm won't work at all without Akonadi running.

I wouldn't advise downloading someone else's KAlarm template calendar (if you can find one to download, which I doubt - the template calendar must have been created by KAlarm in order to work). The whole point of alarm templates is to make it easier to configure alarms which you create frequently, so templates are very much personal to you.

Further to my previous post, please note that newer versions of KAlarm no longer use Akonadi, so the problem is much less likely to occur.