<?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.