The plugin doesn't automatically join visitors to a subscription after payment (paypal)

When someone register a plan, the site doesn't take the user to the paypal's site?

1. I've translated most of the strings in the plugin to Chinese by using PoEditor
(I've also removed the Chinese transaltion file and tried again, but it doesn't help.)
2. I created Access Levels & Subscription Plan
(Access Levels)
(Subscription Plan)
details in subscription plan:
3. These levels and plans are in active and public mode.
4. I choose PayPal Standart Payment as the payment gateway (also tried PayPal Single Payments Gateway)
5. All website visitors register and subscribe plans through the page generated by the membership plugin

My problem is that the plugin doesn't automatically join the website visitors to a subscription after payment done in paypal.
Hence, I should manually add the visitor to the subscription after he/she paid to access the protected content.

I also found the:

I believe the plugin would have the mechanism to done it automatically; however, I can't enable it.
Please help, thanks.