When you are viewing your default template in the Joomla! Template Manager, you will notice a message on the left hand side of the screen which reads: "Menus: Cannot assign default template". Many people seem to believe that this message is reporting an error with one of the templates they have installed, but this is simply not the case at all.

In fact, the message "Cannot assign default template" is just an informational, telling you that you are not able to set the Menu Assignment for the default template, because that template will be used for all menu's unless explicitly overridden by another template.

To fully understand what the message is saying, you need to know that it is possible to assign multiple templates to your Joomla! website, but one (and only one) template will be assigned as the default template. You can determine which template you have set to the default template by opening the Template Manager (Extensions > Template Manager) in your Joomla! administration page. Once the Template Manager screen is visible, you should see a table containing a listing of all the templates installed for your Joomla! application. One of the templates will have a yellow star visible under the 'Default' column of the templates table, that template is the Default Template and will be used to display all of the content in your Joomla! web site.

You can override the default template for specific menu items on your Joomla! web site, by opening one of the other (not the default) templates in the Template Manager, and using the Menu Assignment section to select which menu's / menu item's you wish to display in the alternate template(s).

Using this functionality, your Joomla! web site can have multiple templates based on menu selection by the user. You can use this to make slight modifications to a certain template, enabling you to have a different color scheme based on section or category. You can also have a completely different look for separate sections if that is what you want to do.

So... When you see the "Cannot assign default template" message in your Joomla! template manager, rest assured that there is not an error, or template misconfiguration, you simply cannot assign the default template to specific menus (because it is... the default).