<?php $dictionary['<Custom Module>']['activity_enabled']=true; ?>*Here you'll need to replace <Custom Module> with the same value as step 1.
For anyone still faced with this issue.
Go to custom/Extension/modules/<module_name>/Ext/Vardefs/vardefs.ext.php
and add the following line of code
$dictionary['<module_name>']['activity_enabled']=true;
If you do not have this directory or file then create them.
It is working fine on list view of custom module but not working on detail view just like core modules Leads, Contact, Account etc
It is working fine on list view of custom module but not working on detail view just like core modules Leads, Contact, Account etc