Some time we get the error like
“The default controller for extension “extname” and plugin “Calendar” can not be determined.
Please check for TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin() in your ext_localconf.php”.

Even we have all the correct configurations in ext_localconf.php and ext_tables.php

This is what we have in ext_localconf.php
‘Vendor.’ . $_EXTKEY,
‘Calendar’ => ‘list, show’,
‘Calendar’ => ”,


This is what we have in ext_tables.php
‘Calendar For Dates’,

All the configurations are perfect but still we get the error, why?

Yes there is the reason for it. Suppose we have more than one extension, or more that one plug in in the same extension.
Other extensions or plugins might have had flexforms, and we have added that plugin and configured the flexform.
Now for the same tt_content element we have selected the plugin “Calendar”, and when we select this plugin we dont see any flexform values of
previous plugins.
But in Databse the table tt_content still stores the flexform values of old plug in and this leades to error.

Solution :
1) Go to databse and delete the flexform values for the perticular tt_content element.
2) Delete the old content element and add the new tt_content element.