can we use virtual keyword in abstract class

Yes you can use virtual keyword inside a abstract class.

Because abstract class contain both abstract member and non-abstract member; so when you defining any method using virtual kwyword it means you are giving permission to child class that you can redefine the method in child class when you don't like the behaviour of parent method; so you can use freely any where virtual keyword inside the project as your requirement; see the bellow eg:-

Now Implimenting TestDemo in it's child class:-

Thanks;

Regards:

SK.