After installation, the plugin was showing but has just stopped showing on both desktop site and mobile site.
Please how do I fix this? I use Vanilla version 2.1.1
I suspect you loaded a plugin or theme recently that overrode the entry registration views and the other plugin or theme was out of date and doesn't use 2.1 views. Assuming your are using one of the 3 registrations types that it has been tested with
Basic, Captcha, Approval.
most of the time things don't happen out of thin air, it was possibly due to changes you made. - i.e. incorrectly written or out of date plugin or theme changes.
or you disabled the plugin inadvertently.
whenever you have a mysterious problem....
I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.
Sorry for bordering you. I have just found out that it is the plugin "MailChimp Integration" that I recently installed that disabled "Add registration question".
if you add this line to the views in the mailchimp plugin. you can probably use both plugins. Not a guarantee, just one more reason, not to override view in plugin or theme, if it is not maintained by author through version upgrades.
and so on for all registration views in the mailchimp plugin.