How can my ISP block some sites, even though I’m using a VPN?

How can my ISP block some sites, even though I’m using a VPN?

In my country, internet service providers offer YouTube only, work and learn packages, and Twitter only packages.

I activated YouTube only package and used a VPN. But I can only go to youtube and can't do internet surfing even though I am using a VPN. But as I know, a VPN encrypts the traffic, right? How do service providers block it?

I am using AVAST VPN. How is this technically possible?